This isn't a guide as much as bit of useful information if/when you need to fix your Windows 7 installation.
First - if you can't find your Microsoft product key(s), ProduKey is great product for extracting the keys from your system. This should work for most Microsoft products: http://www.nirsoft.net/utils/product_cd_key_viewer.html
Second - if you can't find your Windows DVD and you need to burn a new one, here's a mostly legal list of downloads:
The last thing I'll mention, you can do an "upgrade" install overtop of any Windows 7 version with the same version. This will "freshen" Windows while leaving all of your programs installed and documents in place.
I'd still recommend a backup before starting anything.