RE: windows.old folder.
On installation, when asked to back up my files I said no, because I had already done that.
Then setup tells me it's going to create a windows.old folder.
It contains:
Program Files, Users, and your old Windows directory and all sub subdirectories.
Total size is 17GB with 62,800 files and 11,850 folders.
All I can say is look through the contents of all the directories and see if there is anything important you want to keep.. Your Program Files directory is useless as all programs need to be re-installed to created the registry entries. Basically all you can look for is documents, pictures, music, etc that you may have forgot about.
I checked the registry to make sure there were no references to it and deleted it, then restarted Windows and ran a defrag. Now my system is much faster.