When I see behavior like that in a PC, I usually yank the HD data cable, and boot the system to a Ubuntu (linux) Live CD.
If the Live CD runs fine, I start looking at it as a hosed OS, hosed HD, or occasionally just a SATA cable gone bad.
If the Live CD also gets you a messed up boot, I would (in no special order)
turn down any overclocking
test the RAM
reset the CMOS to defaults
try and boot up with No keyboard (a stuck key can cause nightmares)
unplug the optical drive power cable or anything else that could cause a weird load on your power supply
unplug any other USB devices like printers or headsets
make sure the motherboard has a working speaker - in case it is throwing out beep-codes on boot up