log files?
Something I use (even on a 250gig hd not anywhere near to being filled) is Crap Cleaner, which cleans ... crap from your computer.
http://filehippo.com/download_ccleaner/
Uncheck anything that you don't understand, or don't know anything about. I clean the everything under the Windows tab EXCEPT the following:
Autocomplete Form History in IE
Old Prefetch Data
The entire Advanced
--I don't touch these, ever.
Under applications, you're pretty free to do whatever. I clean Firefox Cache, and thats about it. Nothing else is really that worrisome to me. In any case, run the Analyze function first and REVIEW. If you see anything you want to keep, find out what found it in the first place and turn it off. Then Analyze again and double check. Use Run Cleaner when you are happy with what is going to be removed.
-Also has a registry cleaner. Use if you are comfortable with cleaning the registry. If you would not normally manually edit the registry by hand, do not use the registry cleaner component of CrapCleaner