Hey people,i finaly Found the answer!!
Now this method worked for me.i don't say that its foolproof solution but its worth trying & befor trying this method update your ATI driver to the latest version(one which is compatible with windows 7).So here we go:
1. set your screen resolution to 1024*768 (lower than highest).to do this simply right click on your desktop,choose screen resolution and change resolution in resolution box.
2.now go to ur ATI catalyst control centre.to do this,right click on desktop and the top most choice is ATI catalyst control centre.
3.choose advanced settings.
4.choose "desktop and display".click on "graphics" on top left corner.
5.now right click on laptop picture in lower bar(in my case there is only on laptop picture in lower bar having "1" in circle on its left top corner)
6. choose "configure..."
7. Now select "Full Screen" from three options.
8. Click "Apply" & then "Ok".
9. Now change back your Screen resolution to original. p.s
:if you are unable to choose "Full Screen" in ATI catalyst control centre then change your screen resolution(first try with decreasing it,if it doesn't help try increasing it) and then try again with ATI catalyst control centre.
Bingo ,we are done!!!
Now try to run your game,it should run in full screen.atleat it did for me.
Dont forget to say thanx if this method worked for you.