I thought very strange that, no matter how much of my data I deleted, I was running out of space constantly these past weeks. Then, I decided to find out how much each directly was using. Due to the lack of "du" in Windows OSs, I thought that Windows Explorer would have similar functionality. I really thought so, because I remember seeing GUIs displaying how much each directory was taking up. Later I would remember that this was KDE or GNOME, not Windows. So, first discovery:
Windows does not ship with any utility to find out how much space is being used on your hard drive.
Having to deal with this limitation, here I go to find out any 3rd party solution rather than right clicking on each of the root-level directories and checking their properties. I found two: diruse.exe (part of Windows 2000 resource kit) and another tool that I can't remember now.
Diruse.exe reports inaccurate values compared to Windows Explorer. It seems it counts the file multiple times for some reason that I didn't care to understand. The graphical tool, which is nice but not free, displayed that I had almost 30Gb that was being used by "unscannable folders" (note: this partition has 120Gb or, in other words, these unscannable folders amount to 25% of it). Go figure what are the unscannable folders. Even being admin, there are some system folders that you don't have access, including one name "System Volume Information".
I gave a few tries to unlock this "System Volume Information", but without being lucky. So I found out what this folder is about: "System Restore" feature - something that I never used and maybe will never user. I followed some instructions on how to disable "System Restore" and clean up this folder and voila: I have 28Gb back.
What surprises me is that:
1. Apparently there is no limit for how much disk this system restore will use.
2. Using Windows cleanup tool, it does not suggest this folder as a way to get disk space back.
3. This folder is locked, so even an admin doesn't find out that it is using lots of disk space. You have to unlock to figure out.
4. Getting to "System Restore" is not intuitive and, once there, it does not say anything about being using disk space. When you disable, there is a warning before disabling, but nothing about the disk space.
5. In the end, what one would do is the same old mantra: reformat and reinstall the system. But, installing from a media that does not have the updates will not help you very much as all the updates will be installed again. And what is the result? System restore points will be logged automatically again, taking up something like 30Gb of disk space.
What's the solution? Unless you understand why that happens, in which case you might get a Vista SP1 or later version that does not require installing new updates, the only solution is getting a larger hard disk or, what is more common, a new computer.