Ahh that pesky Windows.old...
That file gave me a hell of a time when I went to remove it. The best way to get rid of Windows.old (and I wish I knew this back when) is as follows:
Click on Start/All Programs/Accessories/System Tools/Disc Cleanup
When the dialogue box pops up, select the appropriate drive and click OK. The system will do a quick scan and another dialogue box will open. At the bottom left, click Clean up system files.
You'll see another scan like the first one, and then another similar looking dialogue box appears. This time, though, there will be a check-box in the list called Older Versions of Windows. Click it, and click OK. The Disc Cleanup utility will do the rest.