Before doing a reformat I would check a few things. First make sure your CPU fan is still running properly and is not full of dust. Second, run a memory check with memtest
http://www.memtest86.com/ to make sure your ram is ok. CPU overheating and bad memory are two big causes of random lockups. Finally do a full spyware scan.
If that doesn't resolve it then try a reformat. If you still get lockups after that then as Kenshin pointed out then you have a hardware problem. Likely suspects after the ones above are bad power supply or motherboard problems.