I read an article about this some time ago and I wanted to share it with you guys! 
According to ExtremeTech :-
If you were expecting a huge drop in performance as your eyes scanned from the XP to the Vista results, well, surprise! As many a tech analyst predicted, Windows Vista's gaming performance conundrum has largely been solved, and it was mainly due to early graphics drivers.
It took about a year and a half, but the performance gap between Vista and its forerunner has finally evaporated
Read the full article and results here :-
http://www.extremetech.com/article2/0,2845,2303830,00.asp