AVG 7.5 and 8.0 antivirus released an update that removes a critical Windows file. The update marks user32.dll as a Trojan Horse virus. If the file is deleted it may cause your Windows system to continuously reboot.
AVG has released another update to fix this issue. If you have already deleted the file, visit AVG's Support
Forum for information on how to fix this problem.http://www.avg.com/support
More information on this issue can also be found at http://securityandthe.net/2008/11/10/avg-virus-scanner-removes-critical-windows-file/