The .dll had an escalation code written in it. Once given the rights to access a certain control or command promt, it could then escalate the access all the way to your windows registry, where - if they knew how - whomever could have full admin rights to your entire machine. Which is also why the file (and the following ones) was flagged by Norton as a virus.
Unfortunately, it doesn't need access to any command prompts or controls to do damage. IIRC, it has a few routines in it that allow it to do the following:
1. See the processes in the process list that could be seen by the user loading the DLL. If you aren't running EQ as an administrator, that's only yours. This is how they check if any MQ2/SEQ processes were present when you run EQ and connect to P99.
2. Access any file available to the user loading the DLL. If you aren't running EQ as an administrator, that doesn't include the system registry. If you are... it could. This is what they use to check your hard drive for MQ2/SEQ/whatever in obvious places.
Norton, and other antivirus programs, flag the file as suspicious because it contains subroutines that match against potential virus activity. To any heuristics, this file would look suspicious, because... it IS suspicious. The fact that it could be used to hook into anything that's running while you play P99 should be a cause for concern. This is part reasonable security and part tin foil hat.
I mean, yes, it could theoretically wait for you to escalate a process as an administrator and use that process to Do Stuff, but under normal circumstances it will still have access to most of your system without any escalation.