Are you running XP or W2K?
If you are running XP, just do a system restore from the day before or whenever and you should be back in business.
If you need to know how to do a system restore:
Go out to your desktop
Hit F1 (or hit Start-> Help and Support)
Choose the "Undo Changes to your computer with System Restore" (it's on the right hand side)
Choose "restore my computer to an earlier time"
Hit Next
Choose a date, probably yesterday or the day before you made the changes
Pick the system checkpoint from that date and hit next
Should begin the restore process.
If you don't use XP, you can TRY using the "last known good configuration." In Windows 2000, hit F8 before you see the Windows 2000 login screen. Choose "Last Known Good Configuration." The problem with this method is that since you've probably already restarted since your Registry edit, it's already saved this configuration as the last known good configuration.
If you didn't back up your registry, or that key, prior to editing it, you are fucked. If you knew where it was and what the value/text/type was, you can always manually add it, but something tells me that you, like everyone (myself included) wouldn't take the time to document all that shit.