For every system which I do up, typically I will use a freeware called CCleaner. It is basically a freeware which is used to remove unused files and registry liners from the system. Remember as this software deals with the system registry, do remember backup using the option in this freeware.
Hope this tip help as I typically can clear up to a few gigs of crap with the first scan through. It frees up valuable HDD space and speeds up the system.
Post any queries you have on this software if you need to. I'll try my best to help on it.