I'm having pretty much the same problem with avast 4.8 on 2 computers.
After system startup i can access internet websites and/or start downloads.
After some time all downloads will stop (not abort, just stop downloading) and any website called will load forever without any result or error message shown.
Note that i can still access FTP sites without problem.
Also other internet services are still working properly (teamspeak or multiplayer online games).
It seems that only port (or protocol) for internet is blocked.
No matter which browser is used (i tested with IE, Firefox 2 and Firefox 3 Beta)
If the described situation occurs, there is no way to get it running again until you do a complete system reboot - you are not even able to kill system process for the browser from windows task manager.
The same problem also occurs with p2p software (eMule) although there is not used port 80.
If i disable avast (or at least the web shield and p2p shield) the problem seems not to occure (tested only 2 days).
my system is following:
OS: Vista (32bit) with SP1 installed
avast (home edition) v4.8.1201 - VPS: 080606-1, no other anti virus software installed
Firewall: ZoneAlarm (free version) v7.1.254, windows firewall is turned off, windows defender activated
Browsers tested with: Firefox 3.0 RC1, Firefox 2, IE v7.0.6001
i also use a dsl router with integrated firewall, all worked well before avast was updated to 4.8
is there any solution for this problem available? (except disabling web shield and p2p shield)
feel free to ask for more system details if needed