So my computer's running fine, and BAM, it suddenly becomes
nonfunctional. It repeatedly insta-reboots from inside Windows.
Windows will start successfully, but try using it for a minute or two
and the insta-reboot occurs. I figured maybe it's a problem with
Windows, so I tried my Gentoo Linux LiveCD ... not working either. The
computer boots fine and even sometimes gets the kernel booted, but it
quickly freezes after that.

So I'm thinking it may be a hardware problem. I try memtest86 from
within the Gentoo Linux LiveCD. I get millions of errors. At this
point I should probably mention that my memory consists of 2X Crucial
512MB DDR 333Mhz CL2.5 and my motherboard is an Asus A7N8X-X. So I
think one of my memory modules may be bad, so I try running with just
one. I get the errors in memtest. I try running it with just the
other memory module installed. I get the same errors in memtest:
specifically, the Err-Bits seem always to be 00ff0000.

Is it possible for two memory modules to both go bad simultaneously and
in the same fashion? I think it may be *possible*, but it's not
exactly probable. So what could be the problem then? Is my
motherboard borked?

And just to try to head off a question or two, I checked my BIOS
settings and the memory timing is set to 333Mhz.