Instead, Intuit suggests that users try running their existing software under Vista's compatibility mode, which can emulate older flavors of Windows. But it acknowledges that errors will likely still occur.

Microsoft also doesn't escape unscathed. By far, the company has the most applications of any vendor on its own list of verified Vista applications, with 105. But many of its applications that are still popular on Windows XP have not yet been patched for Vista nor officially certified as running fine on Vista. They include all versions of Microsoft Office prior to 2003, all versions of Microsoft Money prior to 2006, versions of Microsoft Works prior to 8 and versions of SQL Server prior to 2005.

A Microsoft spokeswoman said that though it doesn't plan to formally test its older products for Vista-compatibility, most of them should work.

Vista support as a competitive tool

Some vendors view free Vista support as a way to gain customer goodwill. Corel Corp. has released, or plans to release, free Vista updates for the latest version of software available as of Vista's January launch. That includes products as old in its lifecycle as its Corel's Designer Technical Suite 12, which was introduced in February 2005.