Ok I have Windows XP and a LINKSYS router.
Heres what I do to get my server on the internet.
Goto

-->

-->

.
Then Click on something that looks like this.
Your router IP address should be found under "Default Gateway".
Once you found that type the IP in to your Run bar. (go to start --> Run)
That will take you here.

Enter the IP then hit ok.
Now you will get taken to this page.

You may get a popup asking for a Username and Password - LINKSYS by defult the password is admin. there is no user name.
(ask your dad/mom/server_admin for help. if the password admin does not work.)

Once your in hit the Applications & Gaming "Tab".

Customized Applications : HL:CS 1.5
External Port : 27015 to 27015
TCP Protocol : uncheck
UDP Protocol : check
IP Address : your IP on the network
Enable : check
Then hit apply. and your done.