Reinstall from the original win7 disk?
Restore from the restore partition (usually only available on pre-built systems like Dell, Hp...)
Borrow or make another from another pc running the same version you have (home, pro, ultimate...) Your key will still work as long as the version is the right one. 32bit/64bit doesnt matter either.
If you version is a retail (ie - does not have OEM in the key) you can download and burn a new istallation disk from Microsofts DigitalRiver site.