If you have problems after following the instructions from JuninhoSlo, it is possible that because you had 2 firewalls installed on your machine that Avast Internet Security (AIS) got corrupted. If this is the case, try an Avast Repair:
XP Users:
Go to Control Panel > Add/Remove programs > Avast Antivirus.
Scroll down and choose Repair function in the pop-up window.
Reboot.
Vista & Win7 Users:
Control Panel
Programs & Features
Uninstall a Program > Select Avast Antivirus
Click the Change/Remove button and scroll down to Repair
Click Next and follow.
Reboot.
If this doesn’t work, then you will need to uninstall AIS using the Avast Uninstaller Tool http://www.avast.com/uninstall-utility - save it to your desktop. Make sure you have your AIS download file saved somewhere on your HDD before you uninstall. Then follow the instructions to uninstall and do a fresh install, reboot, update your definitions, register your product.