According to an 8 page thread on Nexon Vindictus forums, the Avast v6 is interfering with proper operation of AhnLab HackShield 2 that it uses to keep out hackers.
I’ve confirmed this with an Xp machine with Vindictus and another HackShield enabled game. Uninstalling Avast fixes the issue.
Just a heads up, there are a LOT of reports of conflicts with Hackshiled that are reported with Nexon games that run it on Windows XP. This is just a few samples: