go to Start, Run, and type Regedit. Scroll through the registry till you find Taldren or Interplay, and find SFC2, somewhere in there should be a line that says your CD key. Works for SFC3 anyway, haven't tried it for SFC2. In fact i'd even guess that you could edit the CD key, but don't try it unless you write down your original key first in case it screws up.
Try it
EDIT: I couldn't find the CD key there for SFC2, but maybe i missed it, check yourself...