Huh, sounds like a tough problem to solve...
I have no idea what might be causing this.
Regarding your hypothesis that it has something to do with IPX, I can just say that avast doesn't use IPX in any way - all its communication is TCP-based.
What you may try is disable the avast on-access scanner (or stop the "avast antivirus" service) and see if it has any effect on the problem. If it has, we can move forward...
Thanks
Vlk