Windows XP: Unknown Data Usage

I am currently running Windows XP on a 180gb hard drive = 167gb in real data space.
The problem is i just did a fresh install with a new computer, wiped the hard drive clean, and i only added 114gb of data onto the drive.
However, the problem is it says i only have 16.6 gb of free space, that leaves roughly 37gb if unaccounted space, and i don't believe Windows XP takes up 37gb.
Thank you for your help