Hi Steleten.
Depending on how much ram you have in the pc, its a good idea to pop your windows installation disk in and do a test of the memory, you should do at least three memory test passes to see if you get any errors. If you don`t then I would take a look at the Psu and ask if it feeding the pc with enough stable power.
If you can also check to see how hot your Cpu is getting, And a nice little dust out of the case if it has never been opened for a few years and a good clean out of all the dust helps it breath a bit to keep things cool.
Do a scan of the Hd also making sure there are no errors on it, and perform a malware check, and a virus check.
