PC Freeze

I have a Compaq Presario with Windows ME, I blieve its a 800 mhz processor, and i’m not 100% sure how much RAM as it’s the family’s coumputer, and I don’t use it that much, but my question is the PC locks up. It does when someone is on the computer (opening and running applications, etc.) or when nobody is on it at all (screensaver on) I’m just wondering if this could be a motherboard issue, or a OS issue???