Steam sells Counter-Strike 1.6 as part of the bundle or individually. This gives you:
AAAAA-AAAAA-AAAAA-AAAAA-AAAAA (often used to bypass the prompt in non-Steam versions) How to Activate Your Key Cd Key Counter Strike 1.19
: Add old retail copies to a modern Steam account (if the key hasn't been used before). Steam sells Counter-Strike 1
: Set up "Non-Steam" or legacy LAN environments for older versions like 1.1 or 1.5. Steam Community Cd Key Counter Strike 1.19