Probably Shadow Copies & Restore points, Vista creates them often & they often take up chunks of space once plenty of them are created, so my 1st thought would be to perhaps check that. Use Disk cleanup. I'm sure you probs know where & what but just for if not, then in the search bar after clicking the windows button type 'cleanmgr' without the ' obviously. Once that is done, click the tab "More Options" there you can clean up restore points & Shadow copies, see if that does some justice to your HDD space.

Thanks, I didn't think I would have to run diskclean up so soon! Jeeze, that helped a LOT, I'm not quite where I think I should be in free space, but that did a lot of help. Thanks again.

If the matter hasn't been solved then maybe you had 'Wiped' a partition containing that 100 GB which converted it into the unrecognizable 'Raw' format and is the reason its not being displayed by the OS...Download and try a third party partition manager and see if it finds any unallocated/raw space and convert it into FAT32/NTFS accordingly..

Well it does say solved, however you do have a valid point although I don't think that is what is happening here. Regardless you don't need a third party partition manager in Vista, if you type in compmgmt.msc in the search bar or go to Control Panel > Administrative Tools > Computer Management then click Disk Management, Vista will detect raw data & you will be able to extend your primary partition (with the OS on it) to a larger size by adding the raw data to it.