Welcome to the world of copy protection.
I'm not an expert but from personal knowledge and experience: you're screwed.
I'm guessing that you took the activation key from the bottom of the laptop and tried to use it on your macintel in which case you're pretty much hosed- the key is designed to work with a special OEM version of windows that the manufacturer used (Dell, HP, IBM, etc). For Example, on my Toshiba laptop I didn't have the original restore CD so I just used a full XP version to reinstall and such. When It came time to activate the copy of windows, it denied me because I used a regular version of windows instead of the special Toshiba OEM version.
However, even if its not a company that has a special OEM version you're probably still hosed. Copies of Windows that are purchased can be used on more than one machine (the key I used on this macintel install was first used on the toshiba) but copies of windows that come preinstalled with key stickers on the machine are pretty much tied to that machine's hardware configuration forever and thus non-transferable.