The title explains it all.
I will appreciate any attempts to help me.

Thx.

www.steampowered.com you cant play cs 1.5 on half-life since WON has terminated its service so just download steam from the website then put in your cd-key you bought for half-life and you will be able to play the games you bought.

Wait. What do I do? So I download CS 1.6? After I installed 1.6, I double clicked it and it lead me to a Purchase page.

dont u still have ur cs 1.5 cd-key !?!?
use that key and activate ur hl1s . . . cs1.6 and CZ i think . . . . .
www.steampowered.com
download steam > activate product > enter ur 1.5 cd-key > the games will be enabled in ur game list

Well... correct me if I'm wrong... but I don't see how a mod for CS 1.5 would work on CS 1.6.

How do you even get CS 1.5? It's not in the steam site.
I only have a half-life cd-key.

1.3 ftw :P