Black screen in Windows 7?

I'm having a problem with Windows 7 on my netbook, an Acer Aspire One D270. Sometimes when I want to install a program or when the computer has been left unattended for a few minutes (when the netbook should go into sleep mode), the screen goes black and doesn't clear up again. The screen itself stays on. When I close the lid, the screen lighting turns off and on again when open it again, but it doesn't clear up. The computer will not respond to any commands (Ctrl+Shift+Esc or Ctrl+Alt+Del) but it is clearly still on. When I press the on/off-Button, the netbook goes into hibernation, which is just how I set it up. When it is in hibernation, I can turn it on again and it will boot up and resume Windows where I left off, without any problems. However, this is a highly annoying fix, because it takes a lot of time. The problem occurs in battery mode and when plugged in. Since it just happened twice when I wanted to install a program, I assume it may have something to do with the security-settings, like when you install something in Windows 7, the screen turns black for an instant and then it clears up again to show you a security warning ... except mine doesn't clear up. But then why should that happen in connection with going to sleep after being left unattended?

Any help would be appreciated.