First thing I'd do is disconnect all usb devices and also be sure to check or swap the kweyboard. Can you get into BIOS Setup? Next reseat the RAM sticks. Then one at a time remove add on cards except video card and attempt power up for each one. (best in the order they were added) Keep removing items in this fashion until the beeping stops. This includes all internal drives and switches (other than the power switch) as well. When beeping stops note the last item removed and check condition or replace that item. you can go all the way to just motherboard, power supply, and power switch. If you get all the way to this point then it is one of the two motherboard or power supply.
Note: Try holding power switch for about 4 seconds to turn off without unplugging. (This is not recommended when booted into any operating system as it is the equivalent of unplugging as implemented by motherboard.)
Good luck,