It wouldn't be shadow storage, that would be limited (if set correctly) to probably 10% of the hard drive. I'd suspect TEMPORARY files doing you in OR you're A/V.
I had a similar problem when using BOOTVIS on Vista (which it isn't made for), it was using disk space.
When on XP I also had a problem with Norton's A/V eating up disk space, all unallocated, when it continued to run.
In any event, lets find out WHERE the disk space is being used. So, with that in mind, I'd suggest you change the settings in Windows Explorer. Under TOOLS, FOLDER OPTIONS VIEW tab turn on Show Hidden FIles and off for Hide Extensions for Known File Types and Hide Protected System files. Then look in those folder, SORT by size to put the largest at the top and see if any are large.
Also, get http://www.uderzo.it/main_prod.....index.html it will show the large files and where they are. It will also show UNALLOCATED space by a setting.
Report back please.
Edit by mod: link corrected