There are many infection variants within the same family so I can't tell exactly what this variant does, but generally a "backdoor" serves as a mediator to provide remote access to the system. I recommend running full computer scan and removing all detected threats.
Should the infection be coming back or proves to be difficult to remove, please contact AVG customer care or provide us in this forum or in AVG Forums with more details (mention the full computer scan result in your post and provide us with an msinfo output).
You can try scanning the system using updated AVG Rescue CD first though - should some of the infection component be hidden by a known rootkit, the AVG Rescue CD will remove it. General guidelines on how to remove infections can be found also here.
What is the Backdoor Trojan HUPIGON5.BVBF? Cannot find any info on this one with the BVBF, but have found others. 2013 Free addition found it. Have you any advice other than quarintine.