Here is what I did when I put a new mobo in my computer. It is known that you have to reinstall windows when you change mobos which SUCKS because you do lose a lot when you do that. HOWEVER there is a way to get by that. I went to my local PC Club store and they loaded some data onto a floppy disk that I could load onto my computer. The data that was on that disk put stuff into the registry and made it possible to not have to re-install windows. Only problem is that it has to be done BEFORE you switch out your mobo, so you've missed that step. But for next time you should do that.
Another question, were you wearing a static bracelet while doing your install?
Yes, you can use another windows disk, BUT after 30 days it will force you to register and you risk being caught by Microsoft when you have to call them to get the code you need. You can usually get past that by telling them your computer crashed and you had reinstall windows.
Is the password part that you're talking about a BIOS password or a Windows password?
Sorry for the lengthy reply! One more thing... windows can just shut down like that for a million reasons. It could be your PSU. It could also be your BIOS settings for shutting down if the CPU overheats. Make sure you double check those settings and didn't type in a physical number for overheating, that sometimes can mess it up. Try flashing your BIOS and starting over. You won't lose any windows data doing that. If windows does find enough of it's own drivers to get it started, install the mobo driver disk ASAP.
Again, Sorry for the lengthy reply!! Hope that helps!