Virus removal
One possibility on Windows Me, Windows XP, Windows Vista and Windows 7 is a tool known as System Restore, which restores the registry and critical system files to a previous checkpoint. Often a virus will cause a system to hang, and a subsequent hard reboot will render a system restore point from the same day corrupt. Restore points from previous days should work provided the virus is not designed to corrupt the restore files or also exists in previous restore points. Some viruses, however, disable System Restore and other important tools such as Task Manager and Command Prompt. An example of a virus that does this is CiaDoor. However, many such viruses can be removed by rebooting the computer, entering Windows safe mode, and then using system tools.A virus can modify the registry to do the same even if the administrator is controlling the computer; it blocks all users including the administrator from accessing the tools. The message "Task Manager has been disabled by your administrator" may be displayed, even to the administrator.
Microsoft provide a free anti-malware utility that runs as part of their regular Windows update regime.
