On the Black Screen, you should check your hard drive or fan. For example, if the fan is full of dust and you do not clean it, your PC may run abnormally. So the Black Screen may show up.
On the virus, you can check here to know more about computer and then use the methods metioned in it.
http://ezinearticles.com/?How-to-Secure-Your-PC-Effectively---You-Just-Need-One-Minute-Computer-Knowledge&id=3599864
Good luck!
Wood
January 2010
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