How to use the Disk Cleanup feature to delete the Windows.old folder after you install Windows Vista or 7.
or use Command Prompt
Click Start/ All Programs/ Accessories/right-click “Command Prompt“, and then click Run as administrator.
run following 3 commands on command prompt
takeown /F c:Windows.old* /R /A
Then Type:
cacls c:Windows.old*.* /T /grant administrators:F
Then Type:
rmdir /S /Q c:Windows.old
