1. Boot With a Windows XP CD (the same version as the sticker on the machine) (ie Windows XP Home or Home SP2 etc)
2. Continue with the install using the repair option when it asks weather you want to install a new instance or repair the existing instance.
3. Enter in the CD KEY on the Machine's Sticker.
(It doesn't accept it, you have the wrong verson oem cd)
4. It will eventually be asked to activate your copy of windows. If you are lucky enough for Windows XP to have automatically recognized the nic, you are golden (unless it has been activated to many time). If it doesn't recognize the nic, use the teleophone option and follow the instructions. The voice prompts will give you an authentication code to type in to activate it.