Look at the mainboard to see if there are any bad capacitors, esp around the Memory Banks.

Http://www.badcaps.net for examples....

Yea, I guess it's possible that it could be a dead CMOS battery, but I usually see something like a memory size mismatch type of error when that's happened to me in the past (usually just a pop into CMOS setup and saving it fixes it).  He's getting a NMI parity error.  Although, if the MB had the option between parity and non parity memory (with the default being NOT what he's using), it's possible a bad CMOS battery could cause that....   Anyways, if that doesn't work, read on....

Try taking the DIMM memory module(s) out of your computer's MB, get a can of  compressed air, and blow out the DIM sockets they were in.  Then take a Q-Tip and dip it in some isopropal alcohol, and clean off the edge traces of the DIM memory modules, you can dry they with a paper towel but that shouldn't be necessary. If there is ANY corrosion on those edge traces, lightly sand them off, and carefully inspect the DIMM sockets for corrsion (clean with something like Tarnex, dry).  Reseat the DIM modules into the dim sockets, making sure that they "click" in.  

This is a good time to look around inside the case and do any other cleaning if need be.  Remember to GROUND YOURSELF before working on anything inside the case.  I usually just leave the power cord plugged in, as the cord's ground is directly attached the metal case (touch the case in a bare metal spot, your grounded).  DO NOT use a vaccume cleaner with an upolstry brush attachment, as it can build up a static charge, killing your electronics.

If you are still getting intermittant memory errors, it could actually be a bad memory module (rare, but I've seen it happen, on 2 week old memory no less).  I'm trying to the remember the name of the program I used, off the top of my head, but I can't.  Do a google search for memory testing tools that require to be booted from CD or floppy.  These types of tools DO NOT run on top of an OS per se, and they interact DIRECTLY with each memory module on a hardware level, unlike M$'s Memtest (which is really quite useless).  I think I got the one I used off of sourceforge..net.

Hope this helps you!

since it is very intermittant, evey solution or test will take long.
The memory problem could come from your mobo too, a problem with memory controller, or even cpu.
you can always check for bad caps :   www.badcaps.net
or try running some general tests :
http://www.pc-doctor.com/                                    PC doctor
www.tufftest.com/tt01-lite.htm                        free      TuffTest lite
http://www.passmark.com/download/index.htm                        Burnin test

How old is this machine, and when is the last time you gave it a good cleaning?  Does it happen more often when you turn it on from a "cold" off state, or can it happen even during a "reboot".

If it's very early in the boot process, it sounds to me more like an actual "hard" hardware error, but usually those are not intermittant at all.  A dead/dying CMOS battery usually only causes problems when you've had the machine off for a while.

If it's happening halfway through (or well into) the bootup into windows, it could likely be "dirty" components.  This is why I suggested looking for dust bunnies (previous post), and cleaning up inside your box.  It is possible this is still the problem even if it happens early in the boot process ("ditry" PSU power fluctuations from turning everything on at once).

What happens to a lot on older machines (even as little as 2 years), is that after the computer has been off or rebooted, the fan for the CPU gets stuck on the cowling during startup as it is full of dust/dirt, and the CPU overheats during the bootup of windows.  I have seen this happen more than just a few times....

Don't ignore the power supply and its fans when cleaning,  as that could very well be the problem.  Especially if you've added some hardware since you got the machine.  You may even have to remove it, dissassemble it, and clean it out good (really, it's not that hard, just unplug it first...).  While the power supply rating was good when it was new and clean, the power rating gets "derated" as it gets dirtier.  That's why I always get PSU's with much higher power ratings than needed.  When this happens, it is possible that you are getting power fluctuations during bootup sometimes (everything is being turned on at once) that is causing a power drop to your memory, which could cause the intermittant type of error your seeing.  Putting a Cd/DVD and/or floppies in all drives may cause your error to happen more often.

If it is power supply fluctations, you can test this by using stress testing (burn in) software that basically turns on and intensively uses everything at once (Cd/DVD, HD, Video, memory etc).  Obviously, if it is an intermittant stuck fan like I'm thinking might be, this may not replicate the error.

Obviously you could go broke buying compressed air trying to clean dust bunnies out your whole computer.  If you have an air compressor with a fairly big tank (NOT the tiny portable types), that has the air hose connected at the top (not the bottom), you could fill that up, let it sit in a cool place overnight so that the water vapor condensates to the bottom of the tank.  You can tell if your getting any water vapor out of the nossle by blowing air onto your open palm (if you are, don't use that).  See my previous post about NOT using a vacuume cleaner.  BTW, be careful with how much air pressure your using if your also using it to clean out your CD/DVD drives, as you don't want to damage the stuff inside there.

If its been around a while (or in a dirty environment),  the fans and fan cowlings may actually be encrusted with with dirt, and you'll need something like a stiff toothbrush to get them relatively clean.  You can then use isopropal alcohol & Q-Tips to get it super clean if you want to.

Again, if your going to root around inside your box, remember to always ground yourself before touching any electronic components, or anything attached to them (fans, heatsinks, etc...).  Radio Shack has grounding wrist straps you can buy.  Just make sure that you hook it up to something that is actually grounded (many old houses don't have that ground pin on the AC plug hooked up to an actual ground).

Of course this begs the question, of why the heck don't computer manufacturers put filters and a cheap electronic pressure monometer (to let you know when to clean/change filters) in their boxes?  Having positive air pressure inside the box (versus sucking air through it) also helps to keep dust from settling, and keeps your CD/DVD/Floppy drives much cleaner.

If you are not completely comfortable doing this kind of cleaning project (scared even), I would suggest taking it to someone who is (nerdy relative/friend or computer repair shop), and then see if they will allow you watch them do it.  Then next time, you'll feel more at ease doing this yourself.  A good cleaning can take upwards of a couple of hours if you know what your doing, longer if its your first time.

Repacing the CMOS battery fixed it.  Thank you very much.

your welcome

FYI, I had a similar problem on a rack server. Changing the CMOS didn't work, but cleaning out all the dust did the trick.  Thanks!   (Machine is a Dell Poweredge 2550 that I bought off lease, so it was probably due for some internal cleaning.)