If you're running slow, your problem may more than just low drive space. Unless you're bumping up against your drive space limit, you probably need to look into cleaning your machine up as well (defrag, delete temp files, run spyware/malware/adware scans, etc.)
Probably a little too much for most, but if I ever have somebody come to me with a slow machine (not caused by HW) I just reformat, partiton the drive and reinstall. As crazy as it sounds, it takes less time to start over and do it right than to try to apply Band-Aids to the problem.