Following an especially heavy "Patch Tuesday" this month when Microsoft issued 13 security bulletins, one of the patches proved to be unlucky indeed - Windows XP users reported experiencing a "Blue Screen of Death" after installing it, many were unable to reboot the computer. The update, MS10-015, is one that addresses a kernel vulnerability in 32-bit Windows. There's a fix, but you need your Windows installation CD to boot into the recovery console.
There is a workaround for the vulnerability that's described in the security bulletin for those who decide not to install the update. That involves disabling the NTVDM subsystem.