I have read many an answer about the keypress limit (i.e. "Help! I can only press 2 keys at a time") and it seems that in most cases it is the need for a new keyboard that is the issue.
Well here is a doozy for you then: Me and my girlfriend both have identical motherboards. I have more RAM than her, but that's pretty much the only difference. She had an old ps/2 keyboard and when playing good ol' DOOM, she couldn't strafe and shoot at the same time. The maximum number of keys she could press was 2. "No worries", thought I, "I'll just get her a snazzy new USB keyboard."
So I did, and lo and behold, it had the same problem! "Screw it", thought I, "She can have my keyboard, since I use WASD and the mouse" (She likes the old style keyboard only controls much better) And guess what: Same problem! I can use 3 keys on mine with BOTH keyboards. Left/Right, Alt and Ctrl is no problem for me, but for her it is a big problem.
To summarize: Both keyboards can work with multiple keypresses on mine. Niether work on hers. There is no difference in our versions of XP that I can see, no difference in our BIOS settings that I can see.
It cannot be a hardware issue, so there must be some setting somewhere that dictates key limits. I've been all over google, and there's no help for me there. I've even formatted her computer, out of sheer desperation, and that didn't help. Can you?
UPDATE: It looks like the key limit only applies to combinations of the arrow keys and other keys. If I set the controls to WASD on her computer I can go crazy and press up to 6 keys at a time, but arrows are still a no-no. On my computer, the arrows are not similarly limited. How can this BE? I am flummoxed.