>>>SNIP
>>>>>
>>>>>Can't you use sound instead of messing with the keyboard lights?
>>>>
>>>>What is a computer doesn't have a sound card? My at work doesn't...
>>>
>>>But it can still BEEP.
>>
>>How? I tried ?chr(7) - no sound. Is there another way?
>
>It is possible that you don't even have a speaker plugged in..
>When you turn on the PC do you get the (Post) Double beep?

Nope. No sounds at all.

If the computer does have a sound card, it's easy to catch user's attention: play a "You made an error!" several times. Otherwise I'm not sure: suppose, you have a blind person, who is typing... How would you catch his/her attention without a sound?