Help configuring shared internet conection

For years I’ve been a very satisfied user, but today I started having heavy troubles.

   My computer connects to internet through an USB DSL modem. I share my connection to another computer using LAN. Both computers use Windows XP.

   It used to work, but today, after the light went out, the second computer can't connect to internet unless I disable my firewall. I checked the network rules, they are still there, but for some reason my firewall blocks some traffic from the second computer. I attempted to check the rules, but over the years there are a lot of them.

   Is there a way to get a fresh start and configure it again? Maybe a wizard to tell Comodo Firewall I'm sharing my internet connection? I hate when things stop working withour having modified anything  :embarassed:

I uninstalled IPv6 on both computers (I installed it several days ago and didn’t seem to cause any problem), and now everything seems to be working. I really don’t understand it.