I don't know if this is a Windows Vista-specific trick, or maybe something related to Volume Shadow Copy (the service that supports System Restore), but there seems to be a major discrepancy on my hard drive.
In Windows Explorer, open the C drive, select all, then Alt+Enter to open the properties dialog.
Size: 128gb (137,787,853,411 bytes)
Size on disk: 129gb (138,600,678,813 bytes) (Note: I don't know how there's an odd number of bytes, except that apparently NTFS uses some type of odd clustering strategy - to my knowledge, the "size on disk" field should be a multiple of 4,096, the default NTFS partition cluster size for this size disk).
Now I navigate up to My Computer, right-click on the C drive and choose properties.
Used space: 267gb (287,193,178,112 bytes)
Free space: 664gb (713,020,395,520 bytes)
(This drive is a RAID-0 stripe of two Maxtor 500gb SATA drives)
Now if I set it to display protected OS files and folders, I can come up with a few new items:
4gb in hiberfil.sys (Hibernation support)
4gb in pagefile.sys (Virtual memory swap file)
132gb in \System Volume Information (System Restore)
Looks like I've discovered the reason for the space usage - System Restore. Ugh. Hard to believe I've had this installation of Windows active for about, what, not even a week? And System Restore has managed to take up more space than the files I've installed. The good news is, with it set at 15%, I'll probably only get another 7gb taken. Still.... It's just incredible to me that it's filled so fast.