Hi lilith
The default ports opensim uses are TCP 9000 for the simulator, and UDP 9000 for the region. Both need to be forwarded from your public IP address, towards the IP of the machine running the simulator.
Mind that apart from opening these ports in your router, also the (windows / zonelarm / mcaffee / any other ) firewall needs to allow traffic on these ports !
In case above is all checked, and everybody can connect apart from you ( from within the Local network ) you suffer a NAT loopback issue.
The fix for it is found here :
http://wiki.osgrid.org/index.php/Nat_Loopback