Error in the ax 2012 retail instalation


i have all ready one instance of ax 2009 retail.when i am trying to install ax 2012 retail i am getting the following error.

FullText: System.TypeInitializationException: The type initializer for ‘Microsoft.Dynamics.Setup.MsiRetailBase’ threw an exception. —> Microsoft.Dynamics.Setup.AxSetupException: Portions of Microsoft Dynamics AX are already installed on this computer and they are an older version (5.0.1506.1032) than the one you are now trying to install (6.0.1108.0).
You must uninstall the copy of Microsoft Dynamics AX already installed before proceeding.

Please help how can i slove this error and complete the ax 2012 installation.

Thanks & regards

Bijesh kumar singh