This could also be a hardware problem - check for dust build-ups that could cause over heating in your machine. Also make sure all your cooling fans are clean and running - sometimes worn-out cooling fans run for a while, then freeze up (they appear to be running).
I've had similar problems like yours, caused by over-heating due to dust build-ups or worn out cooling fans. If you machine did overheat, it may be too late and the damage is already done.
Also wouldn't be a bad idea to scan for virus, get one of those online services that will give you a free online scan. If you have windows,
http://www.bitdefender.com is a good one.
http://www.f-prot.com gives away free Linux AV and a program for windows that has one month trial on it.
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