Could be a number of things...
Dust inside the PC
Programmes running in the background
Go into safe mode first and see if it still freezes, press F8 on restart to get into it and mess around in there for a while.
If that is ok, then you might have to do a bit of cleaning.
Removing SPYWARE, Anti-Virus scans, Defrags and scandisks...to name a few.
Basically you need to close as much down as possible, then keep adding till it freezes, then you'll have a better idea of what is causing it.
What OS are you using?