Lets try something I do know about. Cos there aint much else.
Ok tricks, you've run cc cleaner the reg part of that, and the general cleaner, and you have defraged that's a good start.
Ok next, not sure if your running xp or vista, but either way the comp will slow the more things on the hard drive. Xp tends to run better with a hard drive that's only half full, so maybe you need to get some stuff off there.
Two more progs it worth downloading and running are Super anti spyware
and Malware bytes
both free, and both really handy pieces of software.
Next, if xp open the start menu and click run, if vista type straight into the search box on the start menu. Type cmd, when a window opens you should have a prompt that says something along the lines of, c:\blah\blah with a lil flashing cursor. Type in that window chkdsk/f, after a short pause you will get a message saying it cant fun it at the moment would you like to run on restart, type Y, and then restart the computer. It should run through a check.
If all else fails boot it with the OS disk in and run repair.
Without having it right in front of me I'm not sure what else I can offer in the way of advice.
(this advice probably could have come sooner, oh well its here now if anyone needs it)