In case you cannot clean the C:\Windows.old directory using the clean manager (start -> run -> cleanmgr) you can use the following commands to takeover and delete the folder. This mostly saves you between 20GB and 30GB

1. Open CMD.exe as administrator.
2. Take ownership:
   takeown /F c:\Windows.old\* /R /A /D Y
3. Set the ACL so you are allowed to delete the folder and files
   cacls c:\Windows.old\*.* /T /grant administrators:F
4. Delete the folder.
   rmdir /S /Q c:\Windows.old