If you don't have the Win7 disk.
Just download the version you currently have. Don't "upgrade or downgrade" as your key code will not work..
The links are to Microsoft and are downloaded via digital river.. These are ISO files so you can burn them to DVD then install.
Don't do a restore.. Restores suck and put back so much unneeded bloatware.
You need to make sure the settings for your drive in bios should be ACHI.
Do a fresh install so you don't have to fart around with registry settings... Change your power options to NEVER shut off harddrive.
Good luck..