Gaining back space with DISM
I follow this http://www.sevenforums.com/tutorials/122262-windows-7-sp1-disk-cleanup-tool.html by doing the following in cmd windows (run as admin) dism /online /cleanup-image /spsuperseded /hidesp and gain back about 4GB. I don't need to uninstall windows 7 sp1 so had not problem doing this.