They should work if they haven't exceeded the maximum number of activations.
I have learned (from experience LOL) that even if the key has exceeded the maximum number of activations, just do the phone activation and talk to a MS representative. As long as the key is being used on the machine that it was orignally for, you should be good to go.
you don't even need to talk to a rep in my experience, I had the code up on screen, my boss was talking in one ear and I had the phone on the other ear and I pressed the wrong button on the phone.
You get options IIRC "is this the first time you've installed this press 1 for no, 2 for yes" then "is this installed on multiple PCs, press 1 for yes, 2 for no" and I pressed 1 on the second assuming it'd be 1 for no.
Anyway I didn't have time to sit on the phone for 5mins talking it out with them so I hung up, rang back to the automated service, put exactly the same code in but pressed 2 on the second option and it gave me an activation code.
Sometimes knowing how licences work is handy