Windows Not Recognizing Free Space

I’ve been having problems lately with my primary hard drive filling up.  Every once in a while Windows will tell me that I am running out of space on drive C: with only 500mb!  After I cough up my tongue I run CCleaner and do a quick WinDirStat report to see my big offenders.  Nothing glaring ever shows up in the report and I end up just deleting a bunch of old programs and other piddly files that probably only total to 1gb.  But Windows shuts up and I move on.

The last time this happened I took my World of Warcraft directory and manually (and painfully) moved it over to another drive.  This should have freed up 8gb.  It didn’t.

Today I started digging.  Windows correctly sees my drive size as 69.2GB, and reports that I only have 3.15GB free.  That must mean that ~65GB are in use.  However, JDiskReport (and WinDirStat) is showing that only 24gb are in use.

I ran chkdsk to get a raw view of the drive (and to see if there are any errors) and everything came out clean.  It reports that I have 15gb free!

Defrag is scheduled to run weekly and windows tells me that it doesn’t need a defrag.  At this point I’m wondering whether the drive is faulty or do I just need to reformat this drive… (I’d rather not do this, of course)

Any ideas?  I’m out of rope at this point.