I believe it was the Microsoft website where I read this, but it says not to install more than 3GB of RAM for Windows XP 32-bit. Just seeing such a statement without a direct explanation as to why made it sound like a warning. I know about the limitations of 32-bit memory addressing, but I'm wondering if having more RAM than your system can use can cause it to become unstable and, specifically, reboot itself from time to time, or does it just pretend all that extra memory doesn't exist?
I just built a new system for Windows 8 64-bit, so I bought 4 x 4GB of RAM, too. But until I get a product key that actually works (the one that came with mine did not work, and I'm waiting on a reply from the retailer I bought it from), I re-installed XP. My system reboots itself, sometimes before Windows can even finish loading, and sometimes it gets caught in a reboot loop. The lowest amount of RAM I can install is 4GB, which is what I have in there now, as my other three sticks sit in their container eagerly waiting to serve Windows 8.
I feel like my reboot problem is related to something in the hardware, but I'm trying to rule out certain things with advice as I try to decide how to proceed.