Few things I can think of. First I would reset the BIOS to default settings, either through the jumper on the motherboard or by going into the BIOS and pressing whatever button will get you the defaults (usually F10 or F11). Make sure the drives show up correctly afterward and that the boot order is right (HDD first then CDROM). If the boot order is set wrong (to CDROM first for instance) it might cause a disk boot failure if it doesn't find the CD.
It could be a driver issue, maybe the SATA drivers or some other motherboard chipset drivers. This would also explain why it boots fine with the disc in, it may pull the missing or corrupt drivers from the disc to boot up. Go to Asus' website and download any new drivers for your motherboard.
Or it could be an issue with the hard drive, or the master boot partition on it. I don't know if the Vista install disc comes with this utility, but with XP you can access something called the Recovery Console and use it to run chkdsk from a DOS console. This should check the drive for any obvious errors and (maybe, if you're lucky) fix them.
Then I would call whoever you ordered that PC from and tell them to do some more testing next time :bugeye:
