My Computer wont boot up!

Hi Vista Forums.

Recently i have been having trouble with my product key for Vista home premium on my other computer, so i went to a Microsoft help forum and came across a guy who was having the same problems as me, another person had posted a solution saying that he should follow a few steps by going into command prompt and rearming his computer, now, i had a day left before my validation time ran out so i tried the same thing, followed the steps and got a message telling me to restart the computer in order for the changes to take effect, so i thought brilliant! I am going to get this fixed but when i turned the computer back on all i got was a black screen (it looked like the booting screen) with a flashing underscore sign in the top left corner!

Is there anyway i can fix this without having to phone up microsoft to get support?
I need your help asap as its not my computer its another family members.

Thanks

The first thing to try with Black Screen issues is Startup Repair: http://www.vistax64.com/tutorials/91467-startup-repair.html Run it at least 3 times!

If Startup Repair doesn't work, the next thing to try is System Restore: http://www.vistax64.com/tutorials/194765-system-recovery-options.html Restore the system back to a time prior to the black screen issues.

If System Restore doesn't work, then check to see if you can get into Windows in Safe Mode. You do this by rapidly tapping the F8 key just before the Windows splash screen shows up. Select the "Safe Mode" entry and see if you can get into Windows. If so, IMMEDIATELY backup your stuff!!!