Primary Monitor Keeps Changing

Hello, first post here.

I have been having this recurring issue that I find VERY annoying, when I start Windows after the machine being off for a while, maybe a day or two, the primary monitor setting changes to my secondary display. As I do actually have two monitors on the video desk, it is fairly easy (for me) to change the setting to what it needs to be. But I am not the only operator that uses this computer, and most of the volunteers know little when it comes to computers of any kind.
Now you would think that someone with my technical background would have been able to fix this issue, and I can...using a 3rd party solution that costs money. Not the most optimal solution.
So, has anyone encountered this before?

edit: to clarify, I have been having this issue for few years, but I have been the only person using the system. And this was happening on previous systems as well

Specs of the system in question:
Custom Built Desktop;
Windows 7 Professional x64;
AMD Athlon x2 Dual Core Proccessor BE-2400 (64-bit);
2 GB Ram
(Primary) NVIDIA GeForce 8400 GS using version 8.15.11.8593 Drivers;
(Secondary) NVIDIA GeForce 8500 GT using same drivers as above;
Avast! Antivirus;
Using the Windows Firewall, we are on a NAT based network

fireball8931, welcome to the Seven Forums.

Curious as to why you're not running both monitors off of one card, as either one of them is more than capable of running dual monitors.

May or may not help but have you tried any drivers newer than the pre-RTM driver you currently have installed (or is that the default driver that Windows installs?)? The latest is here,

NVIDIA DRIVERS 275.33 WHQL

Are both cards PCI-E or is one PCI-E and one PCI? In either case double check in the bios that it is set right for one of the cards to be set as the primary card.