Halo 2 on XP halp!

I downloaded Halo 2 for Vista, followed all the instructions to patch it for XP, but it still runs so slowly!

I'm talking 5fps-at-best slow! Surely not on a 2GHz, 1GB RAM, 256MB 256-bit Radeon X800 Pro, with 32GB free hard drive space.

I'v tried setting all the detail levels to low, changed the resolution to the lowest possible (800x600) and updated DX9c to latest version.

What am I doing wrong, besides trying to run a game for Vista on XP... I already knew about that

cheers
-anon

Modern gaming pc's still can't handle what the original Xbox could 7 years ago? lulz...