Why is my pc so slow?

Heat is one possibility that comes to mind, what are his temps like?
is his Hdd on the fastest available connection, what speed is the Hdd?
is the hdd full of stuff or just the Operating system?
could be one of many things so we need a little more info
Moto

thermaltake frio heatsink,5200rpm hdd,os and a few games

Eww at the hard drive. If you dont want to spring up for an SSD, get a nice 7200rpm sata III HD with 64MB cache. WD makes a nice 1gb drive that fits the bill.

Yup, that Hdd isn't going to do anyones speeds a favour at all, 7200 as mentioned
I'd still like to know his temperatures, not just the heatsink/fan on the Cpu though  
Hwmonitor or speedfan (or even in bios screen) will tell you his Cpu temp amongst other stuff
Moto

defragment the hard drive...use distrix ultimate disk defrag....works wonder!

I recommend defraggler and run it from a live bootable CD. If you defrag from within windows you will never get a 100% good defrag because of the files that are in use.