i think i know about the Prob !
Latest Direct X 9.0 (August) has some kind of problem Dude :/ . i exactly dont know what ... but when i Installed latest direct X on my XP My Fps Goes Down in many Games Like Gta , Counter strike , Unreal Tournament !!!!
i tested same things on my Friends PC (4 Systems) . there Fps also Goes down :/.
you Can try These Tips may its work for u ? :
1. Uninstall All Display Drivers and Utilities of Display Driver . (restart)
2. Reinstall Original Display Driver from CD . (restart)
3. Make sure, there is no Typical Type setting in your GMA 3100 Display Panel.
4. Always Off "Vertical Sync" from GMA3100 Driver Panel .
5. Install Direct X 9.0c JUNE 2007 . (its awesome work for me)
5. Make sure that your Window is Absolutely Fine and Virus Free !
6. If all of These Tips not Working for you then try to Reinstall the Window from another win XP CD .
7. If Again these Problem Happens, then Tell about these Panchayat's to Your Motherboard Dealer .