If your system hangs after displaying the "splash screen", but before offering BIOS setup, then I wouldn't immediately suspect either the power supply or the hard disk. The first thing I'd check would be to make sure no USB devices (other than kbd/mouse) are attached. Some BIOSes have problems with enumerating USB mass storage devices. Next, I'd try to boot with default CMOS settings. The symptom you describe is usually symptomatic of corrupt CMOS.
Back when I was a BIOS programmer, I created a recovery scheme called "three-boot-fail". After three unsuccessful boot attempts, the next time the system would try the default settings. Some other BIOS brands copied this function, but after three failures would simply overwrite CMOS with the defaults (I left the values intact, but used defaults instead). So, try hitting <Ctrl><Alt><Del> when you see the splash screen (three or four times in a row) and see if that makes a difference.
If not, look for a "BIOS Recovery" or "CMOS Recovery" jumper on the motherboard (refer to your documentation if you have any). Try moving the jumper to the recovery position and booting. If it works, revert to system defaults, move it back, and try again. If you can't find a jumper (or switch), you can remove the the CMOS backup battery (with power off and cord unplugged - important). It is most likely the only coin-cell battery on the motherboard. You'll need to either wait overnight for CMOS to drain, or short out the battery terminals (by pressing on the top metal tab) for a few seconds. Then when you put everything back together and power-up, you should get a power-fail/CMOS-battery error and the settings should revert to their defaults. Then you can get into BIOS Setup to reconfigure the system correctly.
If all that fails to work, I'd disconnect as many devices (such as disk drives) as possible and see if I can get into Setup. Those devices that must be connected (such as memory, video, keyboard), I'd unplug and replug - just in case one has a poor connection. Socketed components do sometimes work their way loose from thermal cycling and re-seating them may do the trick. If you find a configuration that doesn't hang, start reconnecting devices one at a time to isolate the problem component.
I hope that helps,
Eric
