What is the best way to clean up your PC?

Hello all,

I am here to ask what the best way is to "clean up" your PC. I have windows 7 (home premium) running on my computer which I had left with my mum for a while, now its running very slow.

Is there a way to tell what "processes" (in task manager) should not be there?

I use to use ADW cleaner which worker well for me, but old gal removed it because she thought it was bad! HAHA. (Now I cannot find a legit version of ADW Cleaner..)

So how can I remove all unwanted files/processes/nasty things wrong with my computer? Special programs? Picking it out and deleting it manually?

Any & all help is appreciated,

Thank you.