Here's a good page for services. He explains them pretty well and advises which ones are OK to disable. Might help speed things up a bit.
http://www.blackviper.com/WinXP/service411.htmAs far as XP on lower-end machines, ram is the key. 128 is the bare minimum, 256 will actually be enjoyable to use. I have it on a p3 500, and it was miserably slow with 128mb ram. I upgraded to 256 and it runs fine. I don't use it for MAME though, it's a laptop with an 11gb harddrive, it doesn't hold much.
Oh, by the way, speaking of hard drives. CompUSA has 250gb Maxtors for $129. Sale is on till Saturday 6/5.
Paul