NO NO NO...U GUYS HAVE IT ALL WRONG!!!! I had exactly the same errors come up and i was panicking as well, HOWEVER, the CD SOMEHOW recognises that the Demo is installed onyour PC and what you need to do is click on the "Remove All" option to remove it, then click onto setup again and all is fine
This was sending me CRAZI until i found the solution. i had my Win XP cd ready in one hand to format and re-install in need be but luckily i found the solution. hope it helps u all out before its too late!