How to free disk space?

Hi! I am using Windows XP, and finding that gradually the hard disk space is getting used more and more. Used disk clean up facility, Norton antivirus, and Lava Ad-ware Personal repeatedly, but still not able to identify why the space usage is more than what it should have been on the basis of actual downloads. It appears that some files are being copied and hidden somewhere in the hard disk eating up the space. Can anyone help in identifying the cause of this unwanted disk usage and freeing the disk space?

ncdatta