There is no make, model and detail that gives us reason to see why you think it's the power (energy) supply. When power supplies are over taxed the PCs freeze, reboot or shutdown. They have yet to slow down.
Slow down is often due to malware, poor maintenance by the owner (ask them about canned air and if they ask what that is, you know.)
Here's Grif with some malware scanners -> http://forums.cnet.com/7726-6132_102-5098912.html?tag=posts;msg5099421
I have an older computer that I think I need to delete some programs because it is going real slow and doing crazy stuff when I switch windows. I tried to uninstall 1 particular program which I think might be sapping the energy supply, but the "uninstall program" tab is not responding. Could someone please help. thank you