Microsoft Knowledge Base Article 898657 contains the following introduction:

When you try to open a Microsoft Management Console (MMC) snap-in on a computer that is running an x64-based version of Microsoft Windows Server 2003 or Microsoft Windows XP Professional x64 Edition, a dialog box is displayed that prompts you to choose between the 64-bit MMC snap-in and the 32-bit MMC snap-in.

This behavior occurs when both the 64-bit MMC snap-in and the 32-bit MMC snap-in exist for the same task on the same computer that is running an x64-based version of Windows Server 2003 or Windows XP Professional x64 Edition. In this case, you must choose the correct snap-in for the task that you want to perform, and then click OK.