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Many things can be the problem here, as HalT says.
We could go on and list many possibilites but I would direct you to this site -
http://www.viahardware.com/faq/kt7/kt7faq.htm
This just about covers everything.
PS. Forget the PSU as the issue, it doesn't cause slowdowns, just stability problems if it's not strong enough.
Good luck
LH