CD-Key in use by...

Edit: Oddly enough, rebooting my computer corrected the issue, which is rather odd, considering CD-Keys currently in use and such are on the Battle.net servers. Rebooting MY computer shouldn't have done shit.

Well, I'm pretty sure the error is due to you still having a stray connection or something to battle.net. It probably auto-resets it after about 5 minutes normally, but it probably doesn't all of the time. Restarting your computer would kill any connection you have with them.