Post all of your spec. including PSU make and model (if you don't know it you will have to remove the side of the PC and read the label).
Make sure all your graphics drivers are updated (though I doubt it is that, though it might be if you are using XP). My guess is a PSU or CPU/Mobo issue, RAM or GPU would be more likely to cause a blue screen.
EDIT: Re-Reading, does it never happen without the 3G dongle plugged in? If that is the case do a search for a newer driver for that, though again, it should be causing a blue screen not a complete lock up.
EDIT2: While you are having a look at the PSU take a gander at the tops of the capacitors on the motherboard as well. If they are leaking or expanded up then that is likely your problem.
Last edited by jon_hill987; 07-14-2010 at 11:36 AM.