Solution: Yes, you can. If you want to remove that Windows.old folder from of your Windows Vista installation root drive, go to
Start > All Programs > Accessories > System Tools > Disk Cleanup
In "Which files to clean up" dialog > Click on "Files from all users on this computer".
Select the Drive where your Windows.old resides.
In Disk Cleanup dialog, select Previous Windows installation(s) in the "Files to delete" list. Click on OK.
This should clean up the hard disk space occupied by the Windows.old folder.
I verified this on Windows Vista x64.
Refer this.
