If you've been dogfooding Windows 7 like I have, you have an orphaned windows.old directory on your hard drive and you need the space back from Windows.old to get the Windows 7 RTM.

When I Bing-ed for delete windows.old, I found KB Article 930527: How to use the Disk Cleanup feature to delete the Windows.old folder after you install Windows Vista.

 Paraphrasing (and tailoring to my situation Windows.old on the c: drive)

1. Open a cmd window As Administrator
2. takeown /F c:\windows.old\* /R /A
3. cacls c:\windows.old\*.* /T /grant administrators:F (note had to delete, rmdir /q /s, my users directory by hand when cacls kept reporting "The system cannot find the path specified." on various files inside that directory)
4. rmdir /S /Q c:\windows.old\

And now I have 28 gigs back on my c: drive.