I'm a bit confused? What do you mean you're going to reinstall Windows 7 after you revert back to an older version?
If I understand you correctly, the reverting to the older version is temporary. Is that what you were asking?
Anyway, you would need to use the restore DVD that came with the computer to reinstall 7 after you're done with the old version. If the computer didn't come with restore media you need to create your own. If you tell us what type of computer you have I can give you instructions on how to create restore media for your Windows 7 computer.
