Key code doesn't work

Hi all. I have been playing COD2 for quite some time without problems but after 3 days of being offline I tried to enter the server but the message "the key code

is not valid" popped up. I tried different codes and all seems to be valid but when I try to enter the server it still gives the same error. Please help me overcome this issue.

It happens on all servers. I have not purchased the game on steam, I took the install files from a friend and playing it for a few years now without problems