If you used the monitor with WinXP, then it's definitely a 15-pin, VGA style sort of thing. And hopefully no Pentium machine would have built-in EGA anyway. Even a Compaq. =)
But at any rate, EGA uses a different connector, so your machine does not have EGA graphics. It might, however, have a very archaic implementation of VGA. So if you find those resolution and color settings somewhere, go for 640x480 and as few colors as you can. 4-bit color at 640x480 is supported in the brain stem of every color VGA adapter ever built, so that's definitely safe.