If it was a blackscreen reboot I'd agree that it's your power supply, but it's not.
If it's doing a proper shut down and reboot, and it sound slike it might be, I'd check your Windows update settings.
Start -> Control Panel -> Windows Update (Large/Small Icons view) -> Change Settings
Start -> Control Panel -> System and Security (Category view) -> Turn automatic updating on or off
When they're set to "Install updates automatically", Windows downloads the updates, installs them in the background and then reboots your computer if it needs to. I have my update settings set to "Check for updates but let me choose whether to download and install them" because it lets me dictate when and what updates I want installed and if it needs...