With tech stocks unable to mount much of a recovery this afternoon, sellers took charge, leaving the

Nasdaq

below the 4000 level at the close for the first time since Aug. 22.

The Nasdaq finished down 120, or 2.9%, at 3978.4.

TheStreet.com Internet Sector

index ended down 29.38, or 3.5%, at 808.91. Traders took profits on certain issues, while there also was concern over earnings warnings.

National Discount Brokers

(NDB)

closed down $8, or 22%, at $27.81 after it warned that earnings would not meet expectations. The online brokerage and market maker said it was negatively impacted by adverse market conditions during July.

Other online brokerages also fell in sympathy with NDB.

Ameritrade

(AMTD) - Get Report

closed down $1.13, or 5.4%, at $19.69, while

E*Trade

(EGRP)

ended off 81 cents, or 4.3%, at $18.25.

Profit-taking was the order of the day in other Internet stocks.

Juniper Networks

(JNPR) - Get Report

, which has had a tremendous run since the beginning of August, closed down $17.25, or 8%, at $197.63. Other infrastructure plays also suffered.

Redback Networks

(RBAK)

dropped $9.13, or 6.7%, to $127.88, while

Sycamore Networks

(SCMR)

slipped $6.63, or 5.4%, to $116.25.

Copper Mountain Networks

(CMTN)

fell $5.25, or 9.4%, to $50.50.

Among business-to-business stocks,

i2 Technologies

(ITWO)

closed down $9.31, or 5.5%, at $160.50, while

Ariba

(ARBA)

fell $8.13, or 5%, at $153.75, though it traded as high as $166.25.

VerticalNet

(VERT)

dropped $5.50, or 10.2%, to $48.50.

2:23 p.m.: Tech Stocks Riding Bumpy Road Into Weekend

Selling pressures have let up somewhat in the technology sector after the

Nasdaq slipped below the 4000 level for the first time since Aug. 23.

The Nasdaq was down 74.3 to 4024.1 in recent trading after dropping as low as 3988.16.

Traders that established shorts earlier in the week likely covered positions on the slide below 4000.

TheStreet.com Internet Sector

index was down 18.93 to 819.36.

And where else but in this space

yesterday were there some comments from technical analysts saying a dip below the 4000 level was forthcoming? There was no trigger for today's losses, just ongoing uncertainty about market direction heading into what is traditionally a rough period for stocks. The market is seeing some pre-earnings warnings during this confession period, which has put it on the defensive. A failure of the Nasdaq to clear the July 17 high of 4289 has also brought on some technical selling.

Among the day's movers,

Yahoo!

(YHOO)

remained in a weak state, down 3.4%, though buyers stepped in when the stock slipped below the 100 level to 99 3/4. The Internet portal has been beaten up over concerns regarding a slowdown in online advertising.

Online advertising firm

DoubleClick

(DCLK)

was down 5.3%. And ad firm

Engage

(ENGA)

was down a sharp 12%.

Internet infrastructure firms were taking a hit.

Juniper Networks

(JNPR) - Get Report

was down 5.5% in what was most likely profit taking. The stock went from 118 on Aug. 13 to 228 on Sep. 5. Also,

Copper Mountain Networks

(CMTN)

was down 8.4%, its lowest level since April.

Shares of

Retek

(RETK)

were also seeing profit taking after yesterday's 29% gain. The maker of software for the retailing industry was down 3.3%.

TheStreet.com

recently wrote about Retek's

expanded alliance with

IBM

(IBM) - Get Report

.

Also in the business-to-business space, but with different fortunes,

PurchasePro

(PPRO)

was up 7.9%. Keith Jensen, the company's director of investor relations, said that he wasn't aware of anything specific that was causing the companies stock to rise Friday. But a couple of things could be playing into it.

Charles Johnson, Jr., the companies flamboyant founder and ceo, was in New York on Friday filling analysts in on PurchasePro's most recent quarter. Jensen said the company is "very comfortable" with Wall Street projections for the company and that PurchasePro is having a good quarter.

"We only have a couple week's left in the quarter, and we're doing very well," Jensen said.

With that rosy outlook, PurchasePro is also scheduled to give a presentation later today at the prominent Robertson Stephens Internet Conference in San Francisco. That presentation is slated for 3 p.m. PDT, after the market's close.

10:46 a.m.: Online Brokers Having Bad Day

Who could blame investors for not extending yesterday's gains in the

Nasdaq? The market has been anything but rational for much of the holiday-shortened week and early trading today suggested continued uncertainty in the marketplace.

In recent trading, the Nasdaq was down 90.1 to 4008.2.

TheStreet.com Internet Sector

index was 22.11 to 816.13. The market is heading into its confession period when companies pre-announce earnings. It typically performs poorly, a point our own Justin Lahart focused on in a

story earlier this week.

BroadVision

(BVSN) - Get Report

became the latest company to announce a deal with

IBM

(IBM) - Get Report

, though it was not much help to the stock. BroadVision was down 4.1% despite news that IBM would use BroadVision's applications on its RS/6000 servers.

America Online

(AOL)

was slipping 0.1%. AOL and

Time Warner

(TWX)

offered concessions to meet the European Union's concerns about competition in order to win approval for their merger, a spokeswoman for the European Union said.

Earnings-related news also was moving a number of stocks.

CheckFree

(CKFR)

, which provides electronic billing and payment software and services, was 0.5% lower. At an investor conference yesterday, the company said its recent purchase of

TransPoint

would add $500,000 to revenues for its first fiscal quarter, but add 2 cents dilutive to its previously announced expectations of a loss of 10 to 12 cent for the quarter.

First Call/Thomson Financial

had forecast a 10-cent loss for the quarter.

Credit Suisse First Boston

analyst Jim Marks said CheckFree's analyst day was generally upbeat and that the company continues to add customers at a better-than-expected pace. CSFB maintained a buy rating on the stock at a $110 price target.

Online brokerages were getting hurt on a warning from

National Discount Brokers

(NDB)

. The online brokerage and market maker said it expects to report a loss of 6 to 9 cents for its first fiscal quarter ended Aug. 31. That compares to the 14-cent profit expect from First Call. The company said that while its revenues increased over the same period last year, they were negatively impacted by adverse market conditions that the July performance of the company's NASDAQ market maker. It will report its results on Sept. 26. NDB was losing $7.81, or 22%, to $27.94.

Also feeling the hit was

Knight Trading Group

(NITE)

, down $1.63, or 5.2%, to 29.63;

E*Trade

(EGRP)

, down 69 cents, or 3.6%, to $18.38; and

Ameritrade

(AMTD) - Get Report

, down $1.06, or 5.1%, to $19.75.

Several online brokers were upgraded earlier this week.

TheStreet.com

wrote a separate story

on those moves. We also looked at how online brokers have been

rallying, even though little has changed for them.

Santa Cruz Operation

(SCOC)

was down 9 cents, or 2.2%, to $4.13. The company said it would incur a one-time charge of between $5 million and $6 million in the current quarter to pay for the layoff costs of 190 employees. The company said the layoffs were made to meet staffing goals ahead of the sale of its server software and professional services divisions to

Caldera

(CALD)

.

Finally,

priceline.com

(PCLN)

was slipping 6 cents, or 0.2%, to $25.88.

Bloomberg Television

was reporting that priceline was in talks to offer name your own price for wireless telephone service, life insurance, used cars, cruises, Internet services and credit cards, in addition to providing its services in Latin America and Canada.

Staff reporter Joe Bousquin contributed to this story.