Thanks a lot, DELL-RAJESH R!! Problem is solved! emoticon.BigSmile.title
Although the link you gave was for AMD Catalyst Control center, but it lead me to search for ATI catalyst control center for GPU scaling problem and quickly lots of results were returned by Startpage.com. I followed steps given at http://www.hack7mc.com/2010/05/ati-gpu-scaling-removing-the-black-bars.html
I guess, if i had known the correct keywords to search for, problem would have resolved lot time back.
Thank-you for identifying the issue & guiding me to solve it,