Some of it is used by system restore, hibernate, recycle bin and swap space.
For example, if you have 2 gigs of ram, and you have hibernation enabled under power settings in my computer, then it will save 2 gigs of your hard drive space. You also can right click on the recycle bin and take it down from 10% to 3% or so of space. That will also free up some space. Now right click on my computer. Properties, Advanced tab, performance settings, advanced tab, virtual memory. There you can see your swap space used. Set both the initial and maximum to the same size. Make sure its at least the size of the MB of ram you have. So if you have 1 gig of ram, set the paging file to 1024 at least.
As far as the rest of your hard drive goes, if you didn't format the other 50 gigs, you wont see it. You need to format it through control panel, administrative tools, computer management, disk management. Then you will be able to see it under my computer.
If you can see the second partition, d: or whatever, I recommend moving the paging file from c: to d: or whatever the drive letter might be. This should help you free up some space on the C drive.
Also, go into internet explorer's settings and turn down the size for storage of temporary internet files.