If your motherboard was 'broken', you'd be getting blue screens and windows restarts during operation, not necessarily during startup. Also, there's a good chance your PC wouldn't run at all, since the BIOS chip is on your mobo.
If your PC works fine once it's up and running, then it's probably your power supply either on the fritz and/or lacking sufficient power for your configuration. The latter is the case for me, as I am running a 450w with a decent video card and 4 hard drives. Sometimes my power supply doesn't give power to my Windows hard drive on boot and it hangs on the BIOS. A quick push of the reset button usually fixes that. Do restarts remedy your problem? After that, Windows works completely fine for me, so whatever. This only happens about once a month or so.
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