Tell me more. Did you replace all old heatsink compound, check the fans and what about the work (drivers) after you install W7?

W7 is fine but hard on users since Microsoft has never done the post install driver work for you. Microsoft expects us to know this and I don't own your PC so I can't be specific on the drivers except to write you need to do this along with keeping the machine clean and replacing old compound and bad fans.

-> About those drives. I've run into folk that can't wipe a drive. That is, they call the drive bad if they can't format it. Tell more.