PC restart on AVI file playback

From Saturday, my PC restarts on any XVID file playback. I've tried it on MPC and VLC, but both restart my PC.
Here's what happened:

My PC was working fine till Friday night. Everyday, i used to watch movies/series with no issues. On Friday, I was using CCleaner to cleanup my C drive and I also tried its registry cleaner to remove junk entries. After that, I saw one episode of a serial and shut down (not hibernate) my PC. From Saturday, whenever I open an avi file, my PC restarts. Once, there was some error about registry entry of MPCLASSIC and some XVID entry, but on restart Windows fixed it on chkdisk run. Assuming I must have removed some registry entry while using CCleaner (by the way, I chose default options of CCleaner) I reinstalled KL codec, VLC, NVidia drivers, but to no avail.

One more thing, I have Asus 7300 gfx card (the silent version), and when I tried downloading ts drivers (using FireFox), Windows restarted when I clkced download link. The site opened once or twice but download didnt start, so I closed firefox and restarted it, but this time and now every time, whenever I tried to download its driver, PC restarts.

Has anyone faced such a problem? Is this a registry problem, or a hardware prob?
Heres my PC config (4+ yrs old):

AMD Athlon 3000+
1.5 GB DDR 400Mhz RAM
500 GB 7200 Seagate HDD
Asus 7300EN gfx card
MSI Neo4 Platinum
Coolermaster 430W SMPS