Your picture shows 2 picts, however you did not actually chose all files of all types in C:\(as you are thinking). This is something I've always disliked about how Windows shows you the properties when multiple items are selected. It's quite confusing, and really not helpful to read the type/location in the properties window if you have more than 1 file and/or folder selected.
Yours says:
Type: Multiple Types
Location: All in C:\
I just grabbed 2 .doc files on my desktop(and there are more than 2 .doc on my desktop) and I get properties on them it says:
Type: All of type Microsoft Word Document
Location: All in C:\(etc).
Now, I did not have ALL of the .doc files selected in the location specified, nor did I select all of the files in that location. It just generalizes as best as it can and gives you that info. It really is worthless to look at those, except to verify that if you select 10 .doc files, that it doesnt say multiple types.
Now, what you certainly didn't do was select all hidden files. The hidden box and text is greyed out. If the box is grey filled then that means that at least 1 hidden file is selected. In this case it shows that all of the files you selected are read only(probably system files, and not that when I say files, I mean selected them, not in subfolders).
I don't use Vista, and never will, but in XP what you need to do is either go to C:\ and hit CTRL+A to select all files. If you do hot have hidden files displayed, it will probably ask if you want to select hidden files. Click yes and then get properties. The alternate way is to view all hidden/system files and folders, and then get properties.
In XP(and probably even more so in Vista) there are folders that you are not allowed to even view the contents of(one of them is something like c:\system volume information\). Users are not supposed to be looking inside that folder because those are supposed to be for system use only. This was done as a safety precaution so you could not open one of the files to view the contents(locking the file from other programs) and the OS try to write data to it. The OS relies on the fact that it will open those files and use them as it sees fit, and that nothing will ever stop that from happening.
I use a program called disk size manager to look at the folder sizes. It works really well. I don't remember how it handles hidden files, but I always view them anyway. It might give you insight as to what is taking up those 100GB
On another note, try running a chkdsk. That might find that the 100GB of space is marked as used, but is in fact not used. Running chkdsk might free up the space immediately.