Half-life: Counter-strike
 

I've recently got the Half-life: Counter-strike retail version from the net and have upgraded it to ver1005.
I have problem connecting to servers. The WON authentication always seems to fail and this message:"Your CD key cannot be use on the valve system" appears whenever i do so.

What does this means? Do i have to change a CD key? If so, what should i do?
Please advice....:confused:

You are using an illegal cd-key
You connect to their servers
They check your key
They won't let you play.

Buy the game. Simple.