Is it lagging before it freezes or does it just come to an abrupt halt? If it lags, is your task manager showing a program that is memory hogging? P2P will sometimes drag a system way down if it is not regulated.
I had a similar problem after a power outtage. I had to replace the HDD and IDE, no problems now. Mine would hang before it finally froze--eventually it would not even reboot citing "no boot device present".