As you can easily tell on most keyboards, it'll beep when you hold down combinations of keys, such as all four arrow keys. I've seen two answers: ghosting and the BIOS buffer.
This seems to explain it perfectly, but I've seen the argument that it no longer applies to most new keyboards. Indeed the ones at my school are old, and beeped when the arrows, or other sets of keys were pressed. However, the one in my new laptop (and with the price, you'd expect it to follow suit if that were true), does not beep at all. To confirm the problem, I made a simple program to tell me which keys were currently held down. Pressing things like C+F+V yields only two, so it seems to hold. Strangely enough, the four arrow keys worked perfectly. Wikipedia seems to approve this explanation, too.
This also makes sense, especially after the argument that when the computer freezes up, anything you type will cause a beep due to the keystrokes in the buffer still not having been handled. But from the first example, the F+C+V is only three keys. As well, pressing, for example, the numbers atop the letters yielded varying numbers of keys being held down. Sometimes holding 1-6 yielded three of them, and sometimes all six.
What's really going on here?