There have been a million of these before, but there's another fake Windows update via e-mail being circulated.
Here's the obligatory warning: Microsoft never sends out updates via e-mail, so never trust one that comes that way. You want to check for updates? Go to the Windows Update web site.
Sunbelt Software decodes the attachment as IRC.Backdoor.Trojan.
This particular e-mail also has plenty of clues, starting with bad grammar and hysterical warnings ("Please update your P.C. in maximum 12 hours otherwise your Windows is Expired") to just plain awkwardness ("Thanks for microsoft update").
PC Magazine article 1/22/08