CCleaner from Piriform is freeware, cleans up registry, temp files etc.
that and the free version of Avast antivirus and you are pretty good to go.
you can do most of the things you can also do in windows system tools from the program as well.
as another poster also said, getting rid of all the junk toolbars and stuff is a great place to start too, and CCleaner interface does that too, although I prefer to use windows uninstall myself.
My rule.of thumb, if you are prompted to install something don’t do it.