2 Things you will want to preserve: Important data and Software Licenses
Make sure you have your windows 7 activation key and any other keys for installed software. If you can't remember download software to extract the keys. A quick google will bring up things like Belarc advisor and such.
I find it easiest to just copy the entire user folder to an external hard drive. If you want to be 100% sure make a system image in case the reinstall fails.
If you don't have the windows 7 dvd, you can download an iso from the internet and burn it to a bootable CD/DVD or USB