No such email would come from Microsoft. Whatever you do, do not click on any links or downloads.
If you want Windows 7, there are plenty of ways for you to purchase a legitimate copy, from reputable resellers and consumer outlets.
The choice is yours.
At a minimum, I recommend that you be on XP SP3, or Vista SP2, or W7 if you want the latest.
Doc
edit:
Some support cutoff dates:
After April 13, 2010, Microsoft will no longer issue security updates for Vista RTM (release to manufacturing). Vista users are encouraged to upgrade to Vista Service Pack 1 or Service Pack 2, which were issued in March 2008 and May 2009, respectively.
After July 13, 2010, Microsoft will no longer issue support Windows 2000 at all, and will no longer issue security updates for Windows XP SP2.
Users still running Windows XP SP2 are urged to upgrade to SP3 or to Windows 7.