You clean every temp folder out
You switch off system restore on all non-critical partitions, and back off to about 10% space.
You switch off file indexing COMPLETELY .. You don't need on a modest home PC.
You turn off what amounts to about 17 different unwanted services running (Run/Services.msc) (in XP Home and Pro)
You switch OFF all unnecessary running tasks permanently in the programs or in MSConfig (or Registry Tool under start-up items)
You check under .. Start/All Programs/Startup folder and remove Office .. Adobe .. Java and anything else unnecessary.
When you have done that you defrag,, and see how much better it is. It will be.
Post back with the OS you use .. if you did all the rest but the processes, and I'll tell you what to switch off.