Computer not booting up?

I just re-installed my operating system windows Vista. Every thing works great except one thing. After sitting for a while it seems to shut its self off and i can't get it to start to re-boot until I shut of the power. Then I turn on the power and a black screen appears with a flashing cursor in the top right hand corner, and below it says to either click "Return to Window" or "Delete restoration data and proceed to system boot menu, so I push enter and it boots into windows. If i high light "Return to Windows", and push enter it just goes back to a black screen and a flashing cursor.Any help would be appreciated, thanks in advance.