It happens to me as well, your RAM will eventually get used up over the course of playing.
It's really bad on some games and less obvious on others, what I do is that after every 2 hours of playing. Or when i feel that the game is running slow, i reboot the PC and it sorts it all out so it runs fast again.
If your PC is mid-spec u may have to do this more often...
Preferably turn off any antivirus progs will improve performance.
A high virtual memory will help as well, if u want u can try overclocking...
[Edited by EverythingPC, 5/22/2007 8:53:40 AM]
[Edited by EverythingPC, 5/22/2007 12:29:58 PM]
It did help a lot when I turned off the antivirus program and also if i reboot my PC every about 1 1/2 hours