There's an incompatibility problem with Avast and Prisma firewall.
The problem is in the transparent proxy mode of the web shield.
When both are loaded certain programs (ie. Limewire Pro) cannot connect.
I found the solution by uninstalling Prisma and installing ZoneAlarm.
Avast knows about the problem with ZoneAlarm, warns you and gives you the option to turn off the transparent proxy mode (option is normally not available in the free version).
Next, I uninstalled ZoneAlarm then re-installed Prisma and everything appeared to work perfectly again!