Microsoft Windows (no matter what version) is the favorite platform for virusses, trojans, malware and other crap. The way to keep the operating system neat and tidy you should use protection. Protection in the form of anti-virus software and other (required) software to guarantee your survival on the World Wide Web.
Well, protection isn't what it used to be :) . It seems that an update of AVG antivirus recognizes user32.dll as a bit of malicious code. Is this bad you ask? Well.... Windows won't work/boot without it. Ironic?
The solution for this problem is to ask someone with proper knowledge and follow this procedure (dutch forum).