Original cs 1.5 used ports 27005, 27010, 27015 (just for people planning to play, not to host). But since 1.5 will not be playable in the next week or so, you'll have to grab steam @
www.steampowered.com and put up with the horribly coded program! Not to mention the hundreds of thousands of players trying to do the same thing your doing! Sounds like fun? =). I have nothing against 1.6, but steam is horrible.
Anyways open ports:
UDP 1200
UDP 27000 to 27015 inclusive
TCP 27030 to 27039 inclusive