Even more basic; You stated that "my C: drive was already full". Is that really true, in other words is that drive actually at or close to no free space?

If so, you're likely to have all sorts of issues with installation and other basic operating system functions, since that's the only drive where certain of these normal processes can occur.

Please insure you have at least a few hundred MB free on this drive before trying anything else. One way to improve this quickly is to move the Windows Swap file to another drive, since this is often using several hundred MB or more. Then you should look for other data that can be moved or think about re-formatting and installing the entire OS to resolve the problem.

I know what you're facing, I've been dealing with the same issue for months on an old Sony which shipped with a 15GB C drive partition on a 60GB drive. I'm planning to resolve it by starting from scratch when installing a new Windows Vista OS next week.

Bitman