Half Life 1.0 And CD Probs

[mark16]

Can someone please tell me y everytime I try and run a 3rd hand original copy of Half-Life 1.0 on my PC, I can get as far as the options etc, but when selecting my difficulty setting, my CD Drive spins the cd before asking for the disc to be insterted. I know I'm using the right CD-Key, so what am I doing wrong?

Any help would be great

Mark