Note: This procedure will only work with Windows XP Professional.
Click Start -> Run
Type “gpedit.msc” (no quotes) and hit Enter
(this opens the “Group Policy” editor.)
Under “User Configuration” double click “Administrative Templates”
After a few seconds you will see a list of items on the right.
Double click on “Windows Components”
Double click on “Windows Update”
Double click on “Do not display ‘Install Updates and Shutdown’ option in Shut Down Windows dialog box”
Select “Enabled” and click OK
Then close the Group Policy window.
Also there’s …
Official muBlinder Page, Easily Bypass Microsoft Update Genuine Windows Validation (I haven’t tried it)