My pc freezes without warning?

dump any unused programs.. from your add or remove panel..old games, pictures and music burn to disk..do a scan disk and defrag found in your accessory panel..then go to microsoft and get the latest updates for your machine.. reboot the pc,,Have you checked your pc for viruses? if not you can get a free version of windows live one care that will clean your pc ( free for 30 days)..preform regular maintence on your pc..daily do scan disk weekly defrag...this should help but running only 512 memory you may consider getting another stick of memory..good luck and hope this helps