I have a fairly fresh install of Windows Server 2008.
When I right click on the C: drive and view properties, it says I have used 25.2 GB, but when I select all the contents of the C: drive and view properties it says that the size (and Size on disk) are 17 GB.
Any idea where the other 8 GB could have gone?
Are you looking at file size, or file size on disk? Even a file with one character has to take up a block (normally 4k), and this can add up with lots of small files.
Do you have hidden files enabled? You pagefile is probably several GB, if you're not taking that into account.
WinDirStat shows you how much space each folder takes up, give that a try.
Windows 2008 as the preveious versions creates 2 files pagefile.sys and hiberfil.sys each with a default value of 4 GB. These files are placed in the root of your system partition (c:) You can view these files by un-checking the hide system files and clicking on the show hidden files and folders radio button from the ALT-T (Tools Menu) - Folder Options - View Tab in your Windows Explorer window.