Windows is notorious for giving either vague error messages, incorrect error messages, error messages that take you to a webpage with no further info, etc, etc, etc. The error message you show if I had to guess would actually be a permission error. Win 7 has TON more security featuresm where you and teh application may need elevated permission to do things that you used to be able to do no problem. The problem is teh error messages haven't caught up. If you don't have write permissions it can appear to be full. Same goes for some network errors, trying to reach a UNC path for a CIFS share may give you an error saying hte system cannot be found, when its actually permission to the share.
