Can someone help me with a problem HALFLIFE/COUNTERSTRIKE

[DarkBaronSamedi]

My neighboor let me have his Halflife/Counterstrike cd and he gave me the cd key etc When I install it and enter cd key etc Halflife works fine but now that I downloaded CS the cd wont work for either game it tells me it is invalid and the cd is the original one it was not DLed or pirated etc.