Scrubby - unfortunately as you mention, the MS stuff like de-frag and and the clean up don't really do much more that basic diagnostic stuff for your PC. All manner of things from spyware to a registry that needs a good hose taken to it can affect your performance (just as the girlfriend, right
)
That software Biggles has linked is very good at removing loads of garbage, as well as fixing issues with the registry.
I personally use TuneUp utilities, which has all manner of good gear. Have a hunt around on download.com as well.