I've ran into this several times now.
1) Computer craps out because of viruses/malware or dying hard drive.
2) reinstalled with original recovery disc, usually that's the end of it.
3) had to enter product key from the outside of the machine, activation fails
4) calls MSFT, ends up on the phone for about 45 trying to prove that I didn't pirate Windows and they finally gave me a working activation key.
Have had it happen on emachines, Lenovo and Asus. Never on Dell or HP.