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If your computer is acting weird all of the sudden, there is the possibility that you've been infected by a virus.
One quick way to find out is to check the automatically starting programs and services by running msconfig.exe and choosing the last tab. This only works on Windows 98+, though. Windows 95 users can use poledit, which is located on the Win95 CD.
When doing this, check for anything weird, like automatically starting vbs-scripts, programs with unusual names, or programs starting up from unusual places (such as the Recycle Bin).
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