Just realized, if you did a clean install of windows you wouldn't have a windows.old.
I sort of misspoke. You do have to format your drive, in the sense that doing a clean install of Windows will involve blowing away the partition(s) where Windows was previously installed. So backup all your data, run the windows installer from whatever installer media you have, and then when you get to the point where you're choosing partitions delete all your current partitions (unless you had your drive formatted such that you had a separate partition for your data, in which case you could keep that).