at least more so than Windows Explorer, but you need to understand how to 'drill down' and find the apparent missing space.
Does TreeSize list a folder called "System Volume Information"? It may be listed as Access is denied. If so, that holds XP's "System Restore" point backup files and these can take up a lot of disk space. You can see inside by opening Windows Explorer, (My Computer or My Documents will do), and un-hiding "Protected Operating System files" by going to Tools > Folder options, View tab.
If your XP system is otherwise working well, you 'can' delete all System Restore backup files except the latest one. Or, you can disable System Restore then immediately re-enable it. That will delete all of those backup files.
To delete all but the latest. Start > All programs > Accessories > System tools > Disk Cleanup. When it has scanned the compressed files, look at the "More Options" tab.
To disable System Restore, right click My Computer, select Properties. System Restore tab, turn off System Restore, click Apply. Turn SR back on, click Apply. I would create a new SR point immediately.
Other places to look;
1] Windows > Downloaded Program files folder
2] Windows > Winsxs folder. I'm not sure if this is present on XP, (I'm on my Vista rig at the moment). If so, the forum discussion here will explain more.
3] Windows Update folders and files, usually of the form NT$Uninstall or similar. There may be a lot of these update uninstall folders, but they generally do not take up a lot of space. They can be safely moved to a backup if the system is working well.
Others here may have better suggestions.
Alrighty, so this problem started a while back. A good month or so ago, I was surprised to find that I had something like 5 GB left on my 142 GB hard drive, though considering I did have a few .avi's and a large ammount of music I wasn't overly surprised. After deleting a ton files, I was at 17 GB. Not a lot, but manageable. A week or so later, even after only adding a few new albums to my computer, I was mysteriously back at an incredibly low amount of GB.
I downloaded that Treesize program. It says my C Drive has 62,759 MB (which I've calculated to be around 61 GB?) of free space. That's 80 GB unaccounted for and I really need it. I'm sure Treesize isn't perfectly accurate, but that's a lot of free space to mysteriously have vanished. The largest folder is, of course, my music folder. But it's only 31 GB.
I need any advice. Really, any ideas are WELCOME. Especially from people who've had this problem before. I've downloaded a program called CleanUp! and used Windows Clean Up program or whatever, but it's just giving me small ammounts of space. Something else is up here. :/ Something is using well over half of my computer's space.