Power supply turning on and off - Windows skips password

I hope the software aspect of this problem can help shed some light on what the hardware problem can be, but my dad's ASUS desktop has been having this issue where resuming it from sleep causes the power supply to repeatedly turn on and off. The fans start to spin up, then everything shuts down, then the fans spin up, computer shuts down, sometimes for hours without pause until the computer finally decides to resume or the power button is held in to turn it off forcefully. When the computer is force rebooted, the Windows logo screen appears normally, but instead of going to the logon screen and losing all of the work, Windows logs straight into the account (completely skipping the password) and all of the work is still there. Does anybody have any ideas? Would be appreciated. This is a fairly new Windows installation and I've installed a new hard drive (Barracuda Firecuda 2TB HDD).