Oooo, round two.
The first couple of times you booted up it went all the way into Windows? (assuming, of course, the guy is using windows, right?)
Well, it could be a number of problems, none of which should happen on their own and willy-nilly, but...what can you do? /shrugs Maybe when it was wired up the first couple of times, it reset something...stranger things have happened.
I am going to assume that the pc is POSTing and you can get into the BIOS settings.
Double check the BIOS settings to make sure that the hard drive with the OS is on the list of bootable devices. It might be under a sub-heading or something, but I can almost guarantee it
is there. It's like a little priority list of what the pc is supposed to try and boot from. I'm sure you know what I'm talking about and if you don't, you'll find it. Poke around a bit and see if the BIOS recognizes the hard drive correctly. If you don't know much about BIOS settings, I suggest that you just set as much shit as you possibly can to "auto." It's what I do and it hasn't failed me yet.
If that doesn't solve the problem, then it's a hard drive issue (again, assuming that the hard drive isn't a new one, but the same he used before, no?), but before we delve into that I suggest you try all the mobo options first, since it was the source of the initial problem.