There's a couple of things I can think of off-hand. First, different operating systems might make a different... Win2k pro isn't optimized for playing games, although I never had any problems with it when doing so. It might be holding onto memory or caches for longer than you'd like for some reason.
Second, maybe it's a memory leak in a driver for your video card or sound card. I noticed you have an nvidia card, and if you're using their drivers, chances are there won't be any problems... if you're using a release version of the drivers, that is. Beta drivers may cause problems, and I've run into issues with other sets of their drivers which do funny things, like kick me out of games in the title screen...
Anyhow, just two quick thoughts. It's tough to say what the problem might be without a little more information on what hardware and OS is running on the P2. I would think it would be a memory leak, or something of the sort, but it might also be a piece of software running on your duron that's not on your P2 that you may not know is there, eating up CPU cycles... why only after playing Q3, I dunno...
