I thought I'd share what I just found with you, particularly if you are not very familiar with some Vista system functions. If you know how System Restore and Shadow Copies works behind the scenes, you can skip this blog.
I noticed that my hard drive's available space (running Vista) appeared in red from my Computer's Explorer view. So I started removing some unused files. What happened was that the available space increased for a short time, but next time when I checked it, it shrank mysteriously. I changed the folder options to list all hidden folders and files, recorded the size of each folder, but the numbers just didn't add up. I then did Disk CleanUp on the C: drive. That didn't fix the problem. So I turned to the Internet. A quick search helped me find the root cause: the problem had something to do with System Restore and Shadow Copies. To recover the missing space, do a System Restore and Shadow Copies CleanUp, as outlined by a blog at My Digital Life. Since I have had backups by the Windows Home Server, I wasn't hesitant to do that. If you don't have any backup, be aware before clicking the CleanUp button that while getting extra space back you will lose the ability to restore your system to a previsous state where a backup was made. For me I was happy to get that 40GB space back.