Processes being one thing, you might want to consider a cheap video card. It will free up CPU cycles from the On Board Video you currently have. That and more RAM.
Also, I played this game with a P3, 800mhz up until 2006. I had XP Pro and tried SP2 and reverted back to 14 processes on SP1. I pulled 35FPS on a Ti400 and 1st gen DDR-DVI. SP3 IMO, is too much for that PC to ask and you don't need it. If you have the XP home disc, I'd reinstall Home and leave it at SP1. SP2 adds 6-7 processes. I can only imagine what SP3 adds.
1.) Revert from SP3 on that CPU
2.) More RAM
3.) Also get a cheap Video Card. Don't send more than $40 on it.