Maybe you should activate logging in WebShield:
1. Edit <avast>\data\avast4.ini file
2. Find the section [WebScanner]
3. Add the line:
EnableLogging=1
4. Restart Web Shield in XP (terminate and start again) or whole PC in case of Win98
5. Browse (trying to access some webpages)
The log file are <avast>\data\log\ashwebsv.log and ashwebsv.ws.
They would be accessible when WebShield is terminated again.
Post them here or send by mail to rypacek (at) asw.cz
After that, disable the logging to avoid a big log file.
Oh, please try the following:
1. Open the file <avast>\data\avast4.ini, and add the following line to the [WebScanner] section:
ModifyVistaSocketAutoTuning=0
2. Save the changes, restart the computer and see if it makes any difference.
If that does not help, try:
1. Run CMD prompt (as an administrator).
2. Type:
netsh interface tcp set global autotunninglevel=disabled
3. Terminate and restart WebShield. Do you see any difference?