For port forwarding/virtual server to work, you need to specify the destination address. Because IP addresses on basic router networks are dynamically allocated with DHCP and change on a regular basis, you need to give your destination PC a static IP address.

To do this properly though, you need to ensure that the IP address you assign to the PC is not within the DHCP server's scope, in case it accidentally issues the IP to a DHCP client and you get a collision whereby two devices are trying to use the same IP. Locate the DHCP server options within your router's web interface. There will be options somewhere for the IP issue range, that will look something like this:

Start IP: 192.168.0.128
End IP: 192.168.255

The first 3 octets don't really matter (it could be 192.168.1.x or 10.0.0.x for example). The above example is fine as you have all the IPs between 192.168.0.2 and 192.168.0.127 to manually assign. Go into the Network Connections control panel and manually configure an IP for your network connection

• For the IP address, choose one that is outside the DHCP range, but not the IP you access the web interface from. For example, if your DHCP range is configured as 10.0.0.50 to 10.0.0.199 and the router is at 10.0.0.1, assign something like 10.0.0.20. If the range is 192.168.1.1 to 192.168.1.127 and the router is at 192.168.1.254, choose something like 192.168.1.180
• 255.255.255.0 should be fine for the subnet mask
• For the Default Route/Gateway, use the same IP you access the router web interface with. For example, if your router is at http://10.0.0.1, set the IP as 10.0.0.1.
• The same applies to the Primary DNS server. You do not need a secondary.

Now when configuring the virtual server/port forward rule, for the LAN IP, enter the manually assigned IP address.

Admittedly this is a pretty rough post. If you need any more detail anywhere, just ask.