My Cd key Cannot be used on the VALVe System???

I have a problem with esf can u help? well... its this ya c when i try to join i game that i see on this page http://www.gametiger.net/search?game=all&mod=esf or for that matter any game it always says
"Your CD Cannot Be Used On The VALVe System "
Howcome?
Im Going mad killing hte bots every 5 seconds HELP its REALLY annoying! i cant even create a online game i go on lan and make one thats the onlyway i play help me im getting really ;( ;( ;( ;( ;( ;( ;(

Well It Might Be That Someone Else Is Using Ur CD-Key

why not beg your mom/save/work some money so you can buy a new key? isn't it better to use 20$ then to "go mad"?