AntiVir AV software and CCleaner for registry checks clear rubbish up etc; both freeware.
Superlogix Super Utilities Pro excellent for maintenance etc.
Disktrix UltimateDefrag by far the best defragger going (I think there's a free version) to 0% fragmented & options where to place files too; fast and quite hypnotic.

other also use:

Belarc Advisor for info (free).
Sisoft Sandra & Lavalys Everest for hardware info.
XPSmokerPro great for tweeking.

Belarc Advisor,XPSmokerPro,Sisoft Sandra,Lavalys Everest,Superlogix Super Utilities Pro,Disktrix UltimateDefrag, AntiVir AV software,CCleaner