It doesn't make sense either that "only the enter key would work sometimes". Are you testing the same keyboard and mouse across various systems, or using multiple keyboards and mice with multiple systems? Is there any way you can confirm that the PS2 components work correctly on other systems?
As far as I can remember, the PS2 adapter is literally a straight shot out of the super I/O. BIOS really doesn't have any changable settings that could affect that device.
Have you installed drivers for your devices? I think win7 should at least give you basic functionality out of your PS2 devices via inbox drivers, but if you've got some funky PS2 devices, it's always a good idea to make sure you're installing the correct driver(s).
If none of this works, you might want to consider placing a service call if you're still under warranty.
