I have Halo for the Pc and I'm having problems with speed. It always seems to slow down when im fighting. It runs fine when im walking without any enemies around me. Any help?

Specs:
Windows XP Professional
800MHz
64MB Nvidia PCI Card
320 RAM
Sound Blaster Live Sound Card
52X Mitsumi CD-ROM Drive
8/4/32 Plexwriter CD-R Drive

(Have mercy on me. My PC is 3 years old now. ^^)