Monitor Problem

I USE A SAMSUNG 17″ MONITOR FOR MY PC.
FOR LAST FEW DAYS WHEN I TRY TO START
MY PC , THE LOG ON SCREEN APPEAR ONLY AS
A HALF INCH HORIZONTAL LINE WITH ALL PART OF THE SCREEN BLACK . WITHIN THE 1/2″ VISIBLE SCREEN I ONLY SAW THE PICTURE OF MY WINDOWS XP. AFTER REMAINING 1 TO 2 HOURS IN THIS POSITION
THE LOGIN SCREEN SUDDENLY INCREASES TO FULL SIZE BUT START DANCING UPS AND DOWN IN EVERY 2 OR 3 MINUTE NTERVAL .
CHANGING THE RESOLUTION OR UPDATE THE VIDEO DISPLAY ADAPTER HAVE NO RESULT.
PLEASE HELP.
yours.
apr.ch@rediffmail.com