Some programs run very slow.

I have a computer I built myself, with an ASrock P4V88 Motherboard/w 800 MHz FSB, 3GHz Intel P4 Prescott processor, 1 GB of dual channel PC3200 DDR RAM, nvidia FX 5700LE 8x AGP video card/w 128 MB of DDR RAM, Creative Audigy Gamer sound card and a total of 200 GB’s hard drive space,(two IDE hard drives, 1 USB external hard drive). Here is my problem. Most programs (I’m talking mainly games) run extremely well on in, but when I try to run NFS Underground, or any other racing game the program runs but very slowly, and are unplayable. Games like GTA Vice City run flawlessly. When I first installed NFS Underground it ran great, then after a couple of days my problem began. What could be causing this?