737-800 slow speeds

Hi all. Have had to change my version of windows (XP)and just finished installing fs2004 and pmdg 737ng.My problem is that all aircraft including the 737ng seem to run perfect, but they all accelerate and decelerate very slowly. The clock second hand also only moves at about half speed! I tried selecting the simulation rate to x2 and things were better. But this is not normal.Any ideas anyone??All worked fine before.Gfx card is nvidia 7600gs1GB ramxp1800+ amd processorwin XPThanks