It could be. But not too long ago I had a similar problem to yours, and it was the PSU. It was only about a year old PSU, and it still worked, but I would sometimes get a problem when I would start the PC, it would start to fire up for a couple seconds then shut off. I couldn't turn it back on again until I unplugged it and waited about 10seconds. Then I'd plug it in, turn it on, and same thing again. It would do this several times, then it would come on and work fine, for days.
So I bought an enermax 420w and been fine since. One thing you gotta remember, sometimes when PC parts go bad (and ANY of them can, even if new), they won't always completely act as if theyre bad. Like a bad PSU that still gives power, or bad RAM that still lets you run windows, etc.
Of course it could be the mobo, or possibly even something attached to it, like a PCI card or video card, or even fried RAM. From what you're saying, it sounds like it resets before windows even begins to load, so I doubt a windows issue. If you have another comp around you could swap PSU's or other components to troubleshoot. Or you could disconnect devices to see if the problem goes away. Maybe reset your bios to setup defaults, or even reflash the bios. No way of knowing until you try some things. Good luck
Oh and are you overclocking? If so, turn it down to standard. A newer Asus board should give you an error message if overclocking fails, but I've seen PC's do so many strange and unexpected things, almost nothing surprises me.