I have just updated avast on my computer and I have run into the exact same problem.

Avast web shield blocks my internet access, and it also takes 100% of my cpu. When I deactivate it everything comes back to normal.

I would be glad if this problem could be fixed.

To answer your question, I don’t have a firewall.
I have updated avast on a PC I haven’t used for 2 years, so it was a pretty good update, I also updated windows 7. I did a startup scan that found about 30 viruses and I ran CCleaner.
I don’t have any other security programs installed apart from windows malware finder which apparently found a malware.