Don't use the default MAME keys, they suck bad.

If you absolutely need the inputs on your encoder that post CTRL, ALT & SHIFT, map them to coin or player.
Those are much less likely to be pressed in conjunction with other buttons.
If you have an ipac, you don't even have to rewire anything, just reprogram those inputs to something else.
If you have an x-arcade, you can reprogram the buttons, but you'll have to press the button on the back to load them every time you turn on the computer.
I'm not a big fan of the Key Wiz uploader concept, so I just don't use those inputs or use them for administrator buttons.
Solves this problem and many problems you are going to have when you get into other emulators.
Change it now before you get too deep into mapping controls for individual games and setting up other emulators.
I have no idea why people refuse to switch away from the crappy default keys.
