Vista - VM Additions Not Working

I installed VM Additions in my Virtual PC running Win98 First Edition.
Moving the mouse in and out of the virtual machine works fine, as does
dragging and dropping files and resizing the window.
However, when I go to the settings for the VM, I see that the "pointer
integration" box is grayed out and the display settings are all grayed out as
well. Below each grayed out option, there is a message that says "this
option is only available when VM Additions is installed".
I previously had the same problem with "shared folders" although
reinstalling fixed it - but not the other problems.