Follow Black Viper's advice:

Read this one first:
[link|http://www.blackviper.com/WinXP/servicecfg.htm|http://www.blackvipe...XP/servicecfg.htm]

Then this one:
[link|http://www.blackviper.com/WinXP/supertweaks.htm|http://www.blackvipe...P/supertweaks.htm]

There are features and services that are optional with XP, they can be disabled or turned off.

I ran XP Pro in 128M with a 700Mhz Celeron, it ran, but not as fast with a system with more RAM. I had to turn stuff off to get a performance boost.