PC locks up when playing 2 copies of WoW .. but laptop fine

OK, I'm totally mystified here. I have both a PC and a laptop and I've created 4 separate WoW directories on each machine. On the PC, when I run more than one WoW, after about 30 min or so, the machine will hard lock with a blank screen and I have to reset/power off. Nothing gets written to the event log and there is no evidence of the crash/lock up (that I can find) after the reboot. However, on my laptop, I can run 3 instances of WoW with no problem. Here are the specs for each:

PC-
AMD 64 3800+
Nvidia 8800GTX (driver 162.18)
2GB RAM

Laptop-
Intel Core 2 Duo T7200
Nvidia mobile 7950GTX (driver 86.14)
2GB RAM

Any ideas?

i did find some issues... usually centered around the sound card.

make sure you don't have 'background sounds' turned on.
also, make sure to turn off sound hardware emulation.