I remember having that same problem with my first computer. Was a Gateway 486sx and I still have it too. Had same problem running Doom then too, but I never checked to find the limit on simultaneous key strokes. Out of curiousity I just tried 5 keys at once on my old 400mhz Toshiba lap top. I didn't have any problems using the run, look up, change weapon, forward and side step buttons together. I used to play runequake on this computer with a 300 ping and DSTN screen which makes any movement blur the whole screen because the LCD pixels sort of delayed when transisting to different colors. Man I used to get totaly slaughtered but I still loved playing anyway.
MagnesiuM