Hello,
First you should make sure that you uninstalled all PTC Creo 2.0 applications from your computer using Control Panel (also the old PTC Agent).
It is important to read also the requirements of Creo 3.0 (in the installation folder you will find the ReadThisFirst.pdf). It might be an issue with the system.
Please note that Creo 3.0 will not be running on XP.
Thanks,
Alex