1) check that cpu/gpu fans are spinning and cooling fins are clear of dust.
2) look for bulging capacitors on the motherboard.
3) check the hard drive by downloading hddscan and looking for bad sectors
4) download bluescreenview and check what the blue screen of death errors are write them down and post them.
5) download memtest86 and test your ram.
good capacitor:
Bad capacitor:
Really bad capacitors ,line of caps by heat sink: