You are correct. One of the better registry cleaners is CCleaner. It's absolutely free and does an excellent job at cleaning up the registry, cache and cookies. It will even help you get rid of any unneeded programs.
if your rams maxed out some small things to try are
1. disable screen saver.
2. check your virtual memory setting. make sure its not putting to much through your hard drive.
3. disk defrag does speed up your computer but the speed isnt noticable with out some sort of software to read it.
just a few small things. if i knew the specs of your pc i could help you a little more. and theres always over clocking