Mine did this when I forgot to make the game large address aware.
Your setup doesn't sound like that will fix it for you though. Do I understand correctly that your operating system is 32-bit? If it's 64-bit and you have more than 4gb of ram then read this thread.
http://www.subsim.com/radioroom/showthread.php?t=202192
If not, hopefully someone who knows the answer will stumble by.
Welcome to subsim!