After installing Windows XP and all service packs and applications I am likely to use on it, I find it useful to disable services I am not likely to use on it to a bare minimum using MSCONFIG to stop additional CPU/MEM usage going on in the background. Here is a list of those services that I use, based on the fact that I have a cable broadband connection that is always on, and a Soundblaster Live Value Soundcard

Go into Start Menu,

Open RUN command, type MSCONFIG

Goto SERVICES tab, select DISABLE ALL, APPLY, and check the following services to start with Windows and hit APPLY afterwards

DHCP Client - AUTOMATIC 

Plug and Play - AUTOMATIC 

Remote Procedure Call (RPC) - AUTOMATIC 

Windows Audio - AUTOMATIC 

There are also a few other services I use but not all the time, these are listed below and I would either put them on MANUAL if I use them sometimes or DISABLE them completely if I didn't use them at all

Goto START MENU, PROGRAMS, ADMINISTRATIVE TOOLS, SERVICES, and modify the following by right clicking, going to PROPERTIES and change the STARTUP TYPE

Print Spooler - MANUAL  (Only needed if you have a printer)

Server - AUTOMATIC  (Needed for internet connection sharing and access to network drives)

Workstation - AUTOMATIC  (Also needed for internet connection sharing and access to network drives)

That's it, hope this works for you as it gained me at least 50MB of memory back, not to mention a little bit of extra freed CPU time. If you get rid of all STARTUP items from MSCONFIG, you stand to free up even more resources