This is the OLD, "What is a byte?" question/deception that's been plaguing this business since its infancy. Hard drive manufactures market and "HYPE" drive size based on volumes enumerated in base 10. Everywhere else in the computer world from software writers to hardware manufactures it isn't, instead they base on binary values.
So, 1k of memory = 1024 bytes, 1meg of memory = 1,048,576 bytes, 1gig of memory = 1,073,741,824 bytes of storage capacity; 1k of hard drive space = 1,000 bytes, 1meg hard drive space = 1,000,000 bytes, 1gig hard drive space = 1,000,000,000 bytes of storage capacity.
This difference in calculation has been going on from at least the time of sale of the first IBM PC. It didn't matter too much when computers were sold with only 512K of RAM and 30meg hard drives, but now that typical machines boast several hundred GB drives the difference is tremendous.
To calculate the actual space of your 320 gig drives, do to the different definitions of a KB. Divide 320GB (320 x 1,000,000,000) by 1,073,741,824 (the amount of a "true" GB) which =298.023223876953125. So in this example a 320gig drive really only has 298gigs of space because all OSes, software and hardware universally recognize the lower (binary) value for calculating volume size.
So, if you only have a 298gig drive to begin with, then add a diagnostic partition, and a recovery partition (which together take up about 10gigs), to the mix, you only have left 288gigs. Which is the figure you folks have left for your primary Windows partition.
I hope this helps,