If you wan't to change ur CD-key after you wrote it
you(assuming u use Windows): go in the start and choose "Run"
type "regedit" and press enter
u then choose HKEY_CURRENT_USER
then Software then Valve and then Half-life and then Settings
in settings there should be a file called Key ... then you just change the key

I am not sure if this was the answer you where looking for..but anyway here it is (others may use it).