This isn't so much related to ESF particularly and u haven't given more information as to what Half-Life ur using or if ur using Steam etc. so: www.steampowered.com
He is using the WON System, Steam won't say you use a invalid CD-Key. (at least, as far as I know). This thread will help you downloading steam, and installing Earth Special Forces on your steam as well. If you do not have a legal cd-key, you are unable to activate your steam account to be able to download Half-Life or either Half-Life 2 depending on which game you have bought.
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