Can't Build Cavalry

[SandMonkey]

Everytime I go to build cavalry, i got this:

CIVILIZATION3 caused an invalid page fault in
module CIVILIZATION3.EXE at 018f:005537fa.
Registers:
EAX=8de51f89 CS=018f EIP=005537fa EFLGS=00010282
EBX=0000000f SS=0197 ESP=00abf25c EBP=00000008
ECX=0000000d DS=0197 ESI=0ddbec30 FS=113f
EDX=00000007 ES=0197 EDI=0000f1fa GS=0000
Bytes at CS:EIP:
66 39 78 04 75 f8 3b cb 74 13 8b 5e 4c 8b 7c 24 
Stack dump:
0ddbec30 00000000 00000000 00000000 00553c71 0000000f 00000076 00000006 00abf38e 0ddbec30 0ddbc300 00000000 00abf2c0 0059acd3 ffffffff 0040467b

Not that I expect anyone to know what that means, but... what the hell does that mean? I'll try going another turn and then switching to cavalry, but this will be REALLy annoying if i can't build them...