OK, I'm not familiar with Nortons way of doing it but basically the way you need to enable it is as follows.
Firewalls work on 'zones'. YOu have an Internet Zone and a Trusted Zone. Effectively everything the 'internet' side of the routers firewall is considered in the Internet Zone, Everything this side is the trusted zone. You need to ensure that sharing is enabled for the sharing zone and disabled for the internet zone(or anyone will be able to see your stuff!). A lot of people also have firewalls installed on their PCS, these can also get in the way. One of the easiest and safest ways to get this working is to disconnect from the net(actually pull the cable to ensure the router can't do an automatic reconnect). Then disable all firewalls on your PCs and laptops. See if you can share at this point. Then slowly enable each firewall, checking that sharing is still OK before moving onto the next one. When it stops working look in the last enabled firewall for the trusted zone security settings and enable sharing there. When you have got to the point that you can share within your network get ready to reconnect to the net. The very first place you should go is
www.grc.com and complete the 'Shields Up!' test. This will try and connect to your PC and will tell you if it can see anything that it shouldn't be allowed to.
Hope this helps
Let me know if you still have problems