When Microsoft (MSFT) launches Windows 7 next week, its biggest competitor — especially in the multi-user enterprises that are its target market — will not be Linux or Apple’s (AAPL) Mac OS X, but Windows XP.

Eight years after its launch, and nearly three years after Microsoft began shipping Windows Vista (its putative successor), XP is still the operating system most likely to be installed on a new PC in 81% of IT departments, according to a new Forrester Research poll.

Microsoft made it easy for IT decision makers to do what they are naturally predisposed to do — stick with what they know. Steve Balmer is not going make that mistake again, judging from a report published Thursday by Forrester analyst Benjamin Gray.

“Factors are converging,” he writes, “that will provide IT managers with a compelling reason to shake the status quo, finally ending Windows XP’s corporate reign.”

Gray, who last year give his clients five reasons to switch to Vista, now offers the timetable by which Microsoft will make life increasingly difficult for anyone who wants to keep supporting Windows XP.

1. Earlier this year, Windows XP Service Pack 2 entered the extended support phase, meaning it will no longer receive new enhancements.
2. On July 7, 2010, Windows XP Service Pack 3 will follow suit.
3. On April 8, 2014, the extended support phase of both Windows XP SP2 and SP3 will end, and new security updates and patches will no longer be released.
4. Eighteen months after Windows 7 is released or with the release of its first service pack (whichever comes first), the OEM licenses bundled with every PC will no longer carry downgrade rights to Windows XP — meaning that to deploy Windows XP you will have to purchase volume license copies of Windows along with new PCs or use existing, unused Windows volume licenses.

“Already 66% of the firms we recently surveyed expect to migrate to Windows 7 eventually, although most don’t have firm plans yet,” writes Gray, who clearly believes they better get cracking. “That leaves just 27% of organizations that haven’t yet looked at adopting Windows 7 thoroughly enough and 2% that are considering alternatives to Windows 7, namely Windows 8, Mac OS X, and Linux.”

Gray advises managers running Windows XP shops to start preparing now for what could be a long and bumpy upgrade path. For most firms, the processes involved — application compatibility testing, image development, application packaging and testing — can take anywhere from 12 to 18 months.

By which time you can expect Microsoft to start cutting off XP’s air supply.

Gray’s report, “Windows 7 Commercial Adoption Outlook,” is available here for $499.

[Follow Philip Elmer-DeWitt on Twitter @philiped]