backup any important shit you need like documents etc then when you put windows 7 CD into your PC and it boots up into the windows 7 setup, click custom (advanced) and delete all your partitions, then install it in a fresh new one
^ Follow this, Nicknero is a dumbfuck, don't listen to him.
Backup anything you want to keep onto a USB memory stick or a (preferably RW) CD/DVD, and insert your Windows 7 CD and boot from Windows 7 by either a) entering your "Boot from..." menu upon startup (varies from system to system) or b) changing your default boot drive from the BIOS for the time being (which you should only do if the "Boot from..." menu or whatever it may be called on your system doesn't work). Windows installation discs have product keys that usually work twice. So if you've only used the product key once, it should still work. If it doesn't, you can TOTALLY GO LEGITIMATELY BUY (wink wink nudge nudge) Windows and burn it onto a disc and boot from that.
I'm not entirely sure why you would have any issue with Windows 8, as functionally most of your operations will occur in the desktop which, with the exception of a start button (which you can get with modifications, I might add), is identical to the Windows 7 desktop. The Metro UI is only there for Metro apps and startup, so unless you're using Metro apps, you won't even have to worry about it. Windows 8 has better built-in security, and it generally runs smoother since its a newer build. That being said, it's your choice what OS you use. I use Windows 8, but I personally don't support it because of the potential damage it may bring to the gaming industry in the future because the Windows Store system.
Hope that helps,
Ricky