To easily rule out this possibility, Has your HD always been missing this amount of space? Or did you think it wasnt missing that amount of space, or is it a possiblity you were not aware of the missing space untill you needed it.
Often times people buy new hard drives that wont show the real size they are... for instance, I bought a 160 gig a year ago, however my dell said it was like 37gb untill I updated my bios to see bigger drives...
Some bios on some computers cant even see over a 4gb drive...
Can you find the make and model of your Motherboard? if you can I can provide you with a proper link to possibly updated drivers for your bios.
It should not be an issue with windows however. Windows is not that particular about controlling your hardware, usually the bios takes control rather than windows.