Ok for your Firewall. Go to windows security center you will see what firewall are you using as follows here
http://www.dummies.com/how-to/content/the-windows-xp-security-center.html from there it will tell you what firewall are you using by clicking on the down arrow.
Ok before reinstalling avast again, I want you to uninstall avast via add/remove and follow the instructions. Once done, download and run the avast uninstall utility from here
https://www.avast.com/en-nz/uninstall-utility when you run it, the tool will say that this tool will run your computer automatically in safe mode. Click yes and follow the instructions.
Once done, download norton removal tool via here
https://support.norton.com/sp/en/nz/home/current/solutions/kb20080710133834EN_EndUserProfile_en_usManually run your computer in safe mode and run the norton removal tool from above link.
After that download a fresh copy of avast and install it. Once it get installed, it will do a quick start up scan and once it has done it. Restart your computer manually and register your copy of avast.
Also make sure your windows XP has got the latest service pack and fully updated via Microsoft Update.
