Did you know? Windows XP has close to 40 services enabled by default. The vast majority of these are not needed and consume resources and some are even security risks.
Black Viper's site is one of the best windows XP tweak sites I've been able to find it includes:
-Default Service Configurations for Windows XP PRO and HOME.
-Service dependencies, this will tell you what depends on what.
-Various configurations including both the GAMING configuration for those who use their computers for nothing else and the SAFE configuration for those people who simply want to optimize their machines without taking away functionality such as faxing, printing ect.
To change the status of a service make sure you go to START -> RUN and type "services.msc".
NOTE: If you have problems be sure to check the site FAQ and if that does not help you will have to manually reset the services to their defaults unless you can figure out which service(s) are causing the problems.
Edit:
NOTE2: From personal experience I suggest you set the service settings to default before you install any new internal hardware on your machine (ie RAM, Graphics Card or Sound Card). Some of the service settings can cause serious problems when trying to install a new hardware device.