Hi
Having used computers daily for the last 21 years I would personally recommend the following it is free and called CCleaner it is the only one that has never caused any conflicts problems etc and works without slowing anything...and has a fantastic registery cleaner and manager with a super friendly interface.
In those 21 years I have never used any anti - virus or similar and I have never had any problems whatsoever, it really does depend on what you do with your PC, there are 2 resident programs that have seen me clear of any trouble, both freeware one is the ccleaner and the other is malwarebytes, and before anyone criticises I spend all day long surfing and downloading and common sense, lets face facts everyone knows if they are going to download something dubious, so just dont...simples.
Hope you try them, they really do work, on XP, 7 haven't migrated to 8 yet so no comment on that.
Cheers