PC shutdown problem

Win 98 system, fires up but switches off almost immediately unless the bios is entered, when it stays on until the bios is saved and shut down when it switches off. Cards are all seated, power on switch working, keyboard fine, 256mb RAM switched out and checked as working, fans twirling etc, system is only 2 years old. It doesn’t reach the ‘a’ drive so I can’t run a recovery disk. I’ve reset the bios to original settings just in case, but that failed. Unless I’m looking at a motherboard failure or a power supply problem I’m stumped. Can anyone help?