Yeah, I did notice it was outdated. I just updated to the latest version and wouldn't you know it, Avast doesn't have the port open...
As for how I determined it was Avast, I noticed that my MSSQL service wasn't running. Went to start it and Windows kicked back an error telling me to check the Event Log. Checked the error chain and found that
something already had port 1433 open. I then ran NirSoft's CurrPorts and it showed that "AvastSvc.exe" had port 1433 open along with a bunch of other ports in the 14xx range
(really wish I had taken a screenshot...).
In any case, it's fine for the moment. I'll keep on eye on it...
Thanks for your responses!