I use multiple computers, so I kinda have a few spare parts laying around when I upgrade them. The MB is the hardest to change, so I would put that last. The easiest is ram. But that is expensive to buy it and if it doesn't fix it, then have to return it. Maybe you have a local computer store that you can buy a power supply and ram and ask for a return policy. If one fixes it, return the other, if neither fix it, return them both.

I don't think a HD would be a problem that would crash a computer. It might lose your data, but not really crash.

You should do a backup to a good state. So if you can when you get the new computer, do a backup before you do anything, no AV, no spyware, no nothing, just a straight backup. That way if something doesn't work out right (update, install, etc), you can get back to the start. My software tries to do an incremental backup (adds newer parts onto the old ones), so I just renamed the old one so it will be there unmodified so I can always go back to it. Need to do a full Windows install? Bam, just the time to do a restore and you are back in business. Also I would do another seperate backup when you have about 95% of your stuff on. WOW, AV, spyware, other games, etc. That way as long as everything works, you can get back to where you need to be really quickly.

But mostly do an incremental backup about once a week or two. I won't take long and HDs are cheap. I would suggest an external, so almost nothing can crash it.

You might check things that don't take work, just time. Install a video card or hardware temp/load monitor. I use GPU-Z and/or MSI Afterburner and/or Windows 7 gadget "GPU observer". Maybe your computer or video card is overheating. Maybe it is overclocked and your MB doesn't like it. Maybe it only crashes when you are under full load of stuff. Maybe you only crash when you fill up all your RAM. Just turn on computer and don't touch it with power savings turned off. Let it run for 1-2 days. Anything happen? Load up just a game and don't play it. Pick a time for how long to watch it (1-5 hours). Does anything happen? See if there is a demo (cinematic for video or gameplay for CPU and GPU loading) that you can use to load up the CPU so you don't have to be there. Let it run for a time (1-5 hours). Does it work?

Other things that might make things odd, drivers. That is when you can go back to just Windows fresh with no drivers and try stuff. They shouldn't crash. It will run slow as hell, and some, very few but some things might make it crash, but usually just runs slower. Then if things work, then try to update a few drivers. Like maybe sound and MB. Then try again, if things work, update some more. When you get a crash, backup a step and see if it was the newly installed software/drivers.

There are so many things that can go wrong, it is hard to narrow it down. Sorry. I just think of it like programming. Start off with something so small, it can't break. Test the hell out of it and then add more. When something breaks, you know it was in something you just added. Also for me in programming, I like to compartmentalize things, so you have 90% that works. The part that breaks is only in a section that is seperate. For computers, at least you have Windows, it might be slow, but at least it works. Then add more and more to make it work better.

Hope you find something. It sucks to find nothing and always wonder what it was.