Computer freezes randomly

My computer seems to be freezing rather randomly. I can't see any reason that it should be freezing as a result of being overloaded, because I'm not doing anything to overly stress it (nothing that I haven't done tons of times before). For instance, I could just be browsing the internet. Furthermore, when it freezes, my regular tricks don't work. I can't open up the task manager and shut down the process I think is causing the problem. I also can't switch to the desktop or another window. Additionally: I'm not sure, but I think in each case I was able to move the mouse.

After being frozen for a minute or two, I'll force shutdown or my computer will attempt a restart on its own (which perhaps is a result of trying to pull up the task manager). At this point it takes a few minutes just to pass the bios stage.

*It eventually throws out this information:*

Intel(R) Boot Agent GE v1.2.50

Intel(R) Boot Agent PXE Base Code (PXE-2.1 build 086)

CLIENT MAC ADDR:... GUID:...
PXE-E53: No bootfilename received

PXE-MOF: Exiting Intel Boot Agent
No bootable device--insert boot disk and press any key

Additionally: I sometimes get chassis fan not working error before the above information, but I know I have a faulty fan. I don't think this has anything to do with it, but I could be wrong (which is why I'm including it).

The only thing that I've been able to think of to try is to reset bios by removing/reseating the cmos battery.

It starts up fine after doing this. I don't know exactly why this is effective, because I don't yet understand the issue. Obviously, it is only a temporary fix, because this keeps happening. I have to assume the problem is related to the motherboard, but other than that I know nothing.

I was thinking it would be nice if I somehow was able to log what made my computer freeze, but I don't know.

Any help would be greatly appreciated. Thanks ahead of time.

The computer can't find the boot hard drive. It has most likely failed...