Just a heads up on Service Pack 3 and Spyware Doctor.
Spyware Doctor 5.5 or earlier versions of Spyware Doctor may cause Windows XP Service Pack 3 to stop responding when you try to install or to uninstall the service pack
http://support.microsoft.com/kb/951403/en-us
I don't know if any other security software may also be causing problems. I installed it on my systems without any problem (no Spyware Doctor though).