Hello, I'm writing a program and now trying to find a way to check to see if a Windows XP CD-Key is valid. Obviously the algorithm is not known, but is there an API call in Windows that could do this ...

It doesn't actually exist. If it does (I think it actually drops the cd-key and uses an hash of your hardware), it's encrypted.

Using a Windows XP upgrade CD The Boot Camp installation instructions claim that a valid full Windows XP SP2 disc is required and that upgrade discs cannot be used ...