WinDirStat or other similar apps are great! I found out that Windows Media Player was creating thousands of duplicate thumbnails for music/images/videos that resided in the user library folders. It was like 8 GB of thousands of tiny thumbnails. After deleting them I had to use some crazy work around to prevent WMP from indexing the library folders, because if you delete it from the WMP library, it deletes the library folder itself.
As far as losing a gigabyte every 5 minutes; do you have any applications open when you notice this? Adobe apps like Photoshop and Premiere have temp files and/or generate additional cache files when importing/modifying media. Maybe you have an over aggressive backup program creating snapshots? If you're using Vista/7 the Resource Monitor
and checking the disk tab might help when you notice it occuring.
Insert clever text/image here.