chrab, unless you got the media WITH the server you HAVE to uninstall and use media that you have licensed.
I agree with the above consensus anyway. You have no idea what problems you could inherrit. Start from scratch and be done with it. Heck, I'd take the server home and load it up while watching your favorite TV show, then bring it into the office to join it to the domain and DCPROMO if you want to make it a DC for the existing domain.
I hope you find this post helpful.
Regards,
Mark