NIS 2011 Activation problem - Operating System incompatible

I recently purchased and downloaded NIS 2011, when I click to activate I get an error message - product is not compatible with your operating system.  The operating system is Windows XP which I have just installed.  The PC also has Windows 2000 installed.  The NIS 2011 was downloaded in Windows XP.  Would having 2 operating systems cause this problem and if so how do I get around it?

Thanks

Linda