This is not so serious so dont panic.
When you power on your computer, just prior to the windows starting screen make sure your holding down F5 (am sure its that if not try F8 as they did change it a while ago).
This will then give you a set of options. Select to start in safe mode which will start up and log you in as administrator with basic VGA settings.
You can then undo whatever it was that you did. After you have done that simply restart your machine and let it boot up as normal.
Cheers