Hmmm, this is the sort of 'problem' best solved with the PC in front of me ... If you are using XP Pro, send me a "Request for Remote Assistance" and I'll take a look.
You can't do that on XP Home.
I assume the HD is formatted to NTFS and not FAT32 ... NTFS will use 4kb cluster, so for a 1kb file, you will still use 4kb ...
it is a lot better than FAT
1 megabyte = 0.0009765625 gigabyte ... I'll let you do the maths.
You have 'enabled' show all files & folders ... just to "see" if any images from your video file conversions are not hiding.
All your $NtUninstallwmp files can be deleted ... they are the files MS makes when downloading updates so you can uninstall any updates if needed.
I take it you have checked the system for any nasties that may have gobbled up the space.