Just 24 hours after the lavish launch party for Vista , a former Microsoft ( MSFT) engineer has charged that a glitch in the new version of Windows makes it impossible to run hundreds of popular online and downloadable games.

"Hundreds, maybe thousands, of games are broken -- even on Microsoft's own site," says Alex St. John, now CEO of WildTangent, which distributes games to PC manufacturers and via its own Web site.

But the head of Microsoft's casual games unit says St. John's claims "are greatly exaggerated."

St. John, who led Microsoft's efforts to ensure that older games were compatible with Windows 95, said the root of the problem is in Vista's heightened security features.

Simply put, the software doesn't seem to differentiate between noxious "malware" and innocent games.

The software's parental control feature, for example, screens out family-friendly games that aren't rated, he said, adding that getting a grade from the Entertainment Software Rating Board is a hefty expense for smaller developers.

St. John said during an interview that the glitch was discussed publicly at the Computer Games Developers Conference last March but that Microsoft employees in attendance were "dismissive."

Microsoft's studio manager for its Casual Games Group, Chris Early, says there are some games that won't run under Vista, but the number appears to be relatively small. "Of the 101 online games we have on our site, only five don't work under Vista.

"Of the 250 downloadable games, 60% work already and all will work with Vista by the end of February," he says.

Moreover, Microsoft gave developers -- including St. John -- the tools they needed to make their games compatible, Early says.

Those who didn't avail themselves of Microsoft's help "may find that their games don't work when installing under Vista," Early says.