There is a difference between the native "capacity" of a disk and the actual amount of formatted space that a disk provides.
Rather than post a lengthy technical disertation on this, I've included a link below (from Seagate's website, but most HDD manufacturers will have something simliar) that describes this discrepancy in detail.
http://www.seagate.com/support/kb/disc/bytes.html
Now, having said that, a 33Gb short-fall does seem quite large.
As other posters have stated, check your partitions via Control Panel, Disk Management.
Mr Mephisto