Enterprises still using Windows 2000 have another option, of course: migrate now to Windows XP.

"Some were thinking about skipping XP entirely, and moving straight to Vista," said Jump. "But now that will be tricky. We're now generally recommending that they take up XP as soon as possible."

Even that decision, however, comes with potential problems, since Microsoft's current stance is that Windows XP's "mainstream" support will end two years after Vista's introduced. That means companies which migrate now to XP would have to upgrade to Vista some time during 2009 in order to beat the introduction of limited extended support for XP.

Either way, Windows 2000 users have a tough decision to make. And Microsoft's not likely to help them out, Jump said, by extending Windows 2000's paid support, as it did in 2004 for Windows 98.

"I don't think Microsoft will do that," said Jump. "They've been trying to move users from Windows 2000 for a long time."