Sometimes you not only need to open the port, you need to tell the router what do do with a packet that arrives on it. Since your router uses NAT, any machine on the outside doesn't know any of your LAN machines even exists, it only sees your public IP.
There are two ways around this.
First, try putting your PC in the DMZ. Be warned that now you have no pretection whatsoever, besides what you might have on the PC, like Zonealarm.
Second, you've already opened the port, now tell your router to forward it to the PC on the inside. You should be able to find this info in the user manual, at Linksys's website, or if you tell us what model you have, someone here might be able to help. If you go this route, it's best to assign the PC a static IP, otherwise you'll need to re-forward the port to the new internal IP if it ever changes.