Here are some programs to tweak your OS:

XP antispy - block all the worthless services

System Mechanic Professional 5 - get rid of system junk, eliminate duplicate entries, clean up your tracks, stop web popups, fix registry problems, fix broken shortcuts, recover deleted data, manage Windows startup, tweak Windows settings, eliminate spyware, etc.

Tweak XP - optimize [...]

Windows XP has a lot of services, some of which are not needed for normal use. To speed up Windows XP you can disable these services.
Go to Start > Run, type in ’services.msc’.
To turn off a service, double-click on it and select Disabled under Startup type.
Disable the following services:

Alerter

Background Intelligent Transfer Service

ClipBook

Computer Browser

Error Reporting [...]