Well something is eating your memory! Many programs cause a degree of "memory leak" but to get through 320 mb would be impossible. When you say only the normal background programs, are you using an exotic screensaver? I'd say the best thing would be trial by elimination. First close down all background programs except say the antivirus then Ctrl/Alt/Del to see what is still running. Anything unexpected? Then leave your PC for a while and see if the problem arises. If not then add back another background prog and keep doing this until you find the culprit. If all else fails you could use a free memory recovery utility such as RamPage which can be set to automatically recover memory when it drops below a defined level.