I had the exact same problem and these are the steps I used to solve the problem.
When your computer is booting up you have the option to press F2 or F12.
PRESS F2 -Set Up
When your in the Set Up go down to Boot Device Configuration(BDC)
Now under Hard Disk Boot Priority
You Will see first boot device
and boot other device
FOR THE FIRST BOOT DEVICE SELECT "HARD DISK"
After you have made the first boot device hard disk press F10 TO SAVE and RESTART
These are the steps to follow if you have Windows Vista and are dealing with a Black Screen with Blinking underscore. :)