A recent update to antivirus software AVG caused it to falsely identify the file user32.dll as malware on XP systems and prompt users to delete it. Because XP stores critical information in user32.dll—the file contains the graphical elements for things like dialog boxes, according to Microsoft—XP won’t start without it.
AVG claims to have already corrected the false positive in its latest version. For systems that have had the file deleted, AVG provides instructions for booting from the XP CD and using Recovery Console to repair the XP installation. For those without installation media, AVG is providing a utility to restore the file.