do a search for tmp files woukd be a good start, secondly, try and find a free program that shows your hard disk as an image. This will physically show the space taken up. I used to have one for PC, so they are available free. I have one for my mac now also
Under MacOS, you can have the total size of all directories displayed in the file list view and then "drill down" into any implausibly big ones... Maybe the Windows explorer allows a similar tactic.
Alternate possibilities: - Hidden temp files? - Virtual memory swap files? - Erroneously reserved space that isn´t actually used for files? (Disk checking tools should find that.) - Extreme case: A dying harddisk losing large amounts of bad sectors? (Not very likely.)
Finally sorted.. just done a few other Norton scans, then found out what the main problem was. 3GB of music that had been copied and pasted to another folder - probably by my dad when he came round the other night and started messing! Ahh well