to become accustomed to Vista, make sure all is well. Then, when you're happy with the way things are running, delete the windows.old folder.
Type: 'Disk Cleanup' in the search box and click on the disk cleanup icon that appears.
Select 'Files from all users on this computer' and let the cleanup utility do its calculation.
The Windows.Old folder should be amongst the deletion options - possibly as 'discarded windows files'.