ill second the 'CCleaner' (get the Lite version, which is located on the 'other builds' part on there site when attempting to download it) comment cause it's pretty simple and basic so it's unlikely that application would cause any serious harm to your windows install etc, even it's registry cleaning part seems to be pretty safe to my knowledge unlike those more thorough ones which might mess with windows.
but that CCleaner wont help to much if you got spyware/adware etc installed on your PC.
but honestly, if your PC is already loaded down with junk and it's been a long time since you did a clean install (i.e. format(delete all files)/re install windows) your better off doing that and just being careful on what software you install from now on.
in general though just install stuff you need and know to be good and BS free
but if you dont feel like doing the format/re-install of windows stuff... you could try something like 'SUPERAntiSpyware Free Edition' and see what that finds.... but like i said the only sure fire way to make your pc nice and speedy and in good running order is to format/re-install windows!!! i cant stress that enough.... just backup any important data you got before doing it.
but basically what i said in bold is what you need to do if your pc is loaded down with crap and just be careful on what you install in the future.