Although leading security vendors such as Symantec Corp., McAfee Inc. and Trend Micro Inc. have released updates or patches to make some or all of their products Vista-compatible, many others remain deeply entrenched in testing Vista versions of their tools.

Despite numerous complaints from gamers about poor performance on Vista, most applications written for Windows XP run on the new client OS out of the box, with only a few major exceptions, according to Brett Waldman, an analyst at Framingham, Mass.-based IDC.

But not so for security software. Many of the biggest changes made by Microsoft as part of Vista are in areas such as installation and security, including the operating system's user account control, resource protection and protected-mode features. Those changes are more likely to inhibit or cripple security software brought over to Vista than other types of applications.

"It's like they've changed all of the plumbing to make Vista more secure," said Scott Matsumoto, principal architect at Dulles, Va.-based software consultancy Cigital Inc.

Moreover, Microsoft acknowledges that available workarounds -- such as Vista's compatibility mode, which emulates XP and other older versions of Windows so users can run non-Vista-ready applications -- don't work well with software that interacts deeply with the operating system, as antivirus tools and other security programs do.