Hp pc

After a long time in storage I activated the pc (HP) . Blew all the dust out and started up the unit . Original problem was that it would start for 5 seconds and then stop. Followed all the forum suggestions including the psu , cosmos battery ,hd etc Eventually found it to be a start switch was stuck in the on position ( same affect as doing a shut down). Fixed that the proceeded to download a os on to a new hard drive and all went well until it froze on a screen. Entered a icon from the screen then everything went out of kilter. Monitor went to power save mode and stopped communicating to the pc, the usb ports went dead, and I get the long beep ( probably 3 seconds) repeated . Followed forum suggestions of removing parts including the cosmos battery and then replacing part at a time . NO success as it still beeps I need confirmation that the problem will be in the mother board before I spend money on a new unit.