First off, how far into the Windows process does it get before you have to reboot? Do you see the Windows logo at all?
I can't see why you can't boot from a CD if you are able to start booting Windows, but obviously I'm only going by what you're telling me.
Normally pressing Del when you are starting up (well before you boot Windows) should get you into the BIOS - from there you can tell the BIOS to boot from CD first, then Windows.
Secondly, how valuable is the stuff on the Windows partition?
You've got a few choices that vary in terms of the work and luck required.
1. Boot into Safe Mode (F8 when you see the Windows screen, then a bunch of startup options appear). This will give you some debugging and load a minimal Windows environment so you can isolate the source. Perhaps the resets will still occur, but perhaps you'll get to actually read the error message.
The other choices depend on booting from a CD. Not sure why you can't get into the BIOS and change the boot order as it always starts up before Windows does.
Anyway:
2. Reinstall Windows on the old partition - replaces the contents of C:/Windows with all the Windows-related files - including the registry - which means you will need to reinstall everything, but you won't lose any data outside of the Windows directory. This may fix the problem or it may not, but it could give you the time to backup your stuff and reformat.
3. Wipe the partition (Windows install CD or your average Linux live CD can help you with this) and reinstall Windows - clears everything on that partition. All the stuff that is on there will be gone, but the problem will be gone.
What concerns me is the error message. Do you have the original CD for the motherboard drivers? What often happens is that Windows installs the best match it can find for a piece of hardware, which may work fine, but is hardly foolproof. The best thing to install is the driver on the CD, as that is what the manufacturer recommends you use. If there is an updated version of that driver it would appear in Windows Update, but aside from that (unless you're using Vista - for example the nVidia drivers have some bugs which are affecting some games in Vista and Vista only) that the CD drivers are sufficient.