The error messages are all in Korean, but the most common error codes are 110 and 114.
GameGuard is incompatible with some anti-virus software. A Google search for "GameGuard Avast", for instance, turns up pages that explain how to add exclusions to Avast!, and this apparently works for Ijji, but it does not work for Aion.
The problem is that Avast! thinks GameGuard is a virus (rootkit), and GameGuard thinks Avast! is a cheating attempt, so they block each other.
The bad part is that when GameGuard exits after the error message, it seems to do some damage to the operating system on it's way out (at least, in Vista it does). After a failed load, you will be unable to browse any websites, cannot execute the Task Manager, and other instabilities can occur including a complete lockup. Rebooting does fix it, but so long as Avast! is active, you cannot run Aion.
The only "fix", at this time, for Aion is to disable Avast! when you want to play Aion. NCSoft is aware of this issue, but there is no promised fix. There seems to be no problems re-enabling Avast! after you are finished playing.
As of NA / EU Launch, NCSoft has decided to temporarily remove GameGuard from Aion. There is nothing saying they won't add it back in at some latter point, but for the Live Launch they removed it. In general, this was greeted with much cheering, as it is unusal for NCSoft actually listen to the multitude of voices that had been begging them to get rid of this severely deficient program.
There has been no statement as to when or if NCSoft will put GameGuard back into Aion.