Join the big show, and watch your stock drop. That's what happened Friday to e-commerce software-maker

Ariba

(ARBA)

, which joined the

Nasdaq 100 index

and saw its shares drop nearly 10% all in the same day.

Of course, that might also be because Ariba jumped 13% yesterday and some investors found the temptation to profit irresistible. Rival

Commerce One

(CMRC)

saw a dip as well, as its shares closed down more than 5%. Historically, the two stocks often chase each other up and down.

Ariba reported last week that its revenue climbed sharply in the latest quarter and posted a smaller-than-anticipated operating loss. But overall, Ariba is down 11% for the week.

Nortel

(NT)

, the officially designated underachiever of the week, closed down more than 6% to $42.55. For the week, Nortel dropped 35%.

4:43 p.m.: Old Tech Friends Spend Day Happily in the Green

The feeling today was that old friends are the best friends -- at least when it comes to tech stocks.

Technology stocks went higher, apparently helped by

JDS Uniphase's

(JDSU)

good report Thursday. Then tech stocks lost their surge, and investors sought refuge with some old familiar names like

Hewlett-Packard

(HWP)

,

Microsoft

(MSFT) - Get Report

and

Intel

(INTC) - Get Report

.

"I think a lot of fears were relieved with JDS. And they like the big tech names," said Tim Grazioso, manager of

Nasdaq trading at

Cantor Fitzgerald

.

TheStreet Recommends

Intel ended up 3.8%, Hewlett-Packard ended 1.4% higher and Microsoft gained 5%. Fellow technology bellwether

Sun Microsystems

(SUNW) - Get Report

also was up today, about 1.2%.

With semiconductors, however, volatility continues apace with semiconductors -- and it might stay that way for a while.

Many semiconductor stocks slid, as did the

Philadelphia Stock Exchange Semiconductor Index

, which dipped 2.9%.

Chip-maker

Teradyne

(TER) - Get Report

was one of the day's biggest losers, dropping 9.5% after a downgrade from buy to outperform by

Lehman Brothers

.

Micron Technology

(MU) - Get Report

dropped 4.4% after

Robertson Stephens

downgraded the stock Friday from buy to attractive. Micron was downgraded Wednesday by

Chase Hambrecht & Quist

and

Banc of America Securities

.

Investors rotated into chip giants Intel and

Advanced Micro Devices

(AMD) - Get Report

, with AMD gaining more than 0.6%, finding comfort in familiar names in what has been a volatile semiconductor market in recent weeks.

It's a pattern that's likely to continue as the market strives for a way to measure performance in semiconductor stocks, analysts said.

Some of the measures used in the past -- for example, DRAM prices or cell phone growth -- no longer work as an effective measurement for most of the chipmakers, said analyst Susan Billat of Robertson Stephens, but some investors still "cling to the familiar."

Until the market can find ways to measure performance for each of the different parts of the sector, the volatility will continue, said analyst Drew Peck of

SG Cowen

. "Problems with one company don¿t necessarily mean problems with another, because the end markets are very different."