Although system restore is turned off, you may have to ‘unallocate’ the space or, at least, some of it.

Open an elevated command prompt by clicking the Windows Orb (Start) > All Programs > Accessories and right-click Command Prompt, then ‘Run as Administrator’. To display the current parameters type (or copy and paste) “vssadmin list shadowstorage” (not the quotes but the spaces) and press Enter.

Change the allocation by typing (or copy and paste) “vssadmin resize shadowstorage /on=c: /for=c: /maxsize=0GB” (not the quotes but the spaces) and press Enter.

Then run the diskpart command after booting from the Vista DVD.