upgrade from standard to enterprise

I have a windows 2003 standard edition R2 SP2 and i need to upgrade it to the enterprise edition because i have 8GB RAM physically installed and the standard edition only recognizes 4GB. When I launch the installer of the enterprise CD i get the following error:
?Setup cannot run in program compatibility mode. To run Setup normally, go to My Computer, double-click the CD icon, and then double-click the Setup.exe file?.

Please note that I tried to run setup by double clicking the executable but I received the same error.

Any advise that might help me launch the installer?

Thanks in advance.