GTA 4 help

Hello,

hey i have just installed GTA IV on my pc , my configs are as follows,

windows xp service pack 3
2.2 core 2 duo processor,
3 GB ram,
nvidia GTS 250 Graphics card

with these specs i cant run the game on full graphics, the option is somehow locked it wont change from medium, i cant really believe that GTS 250 cant run this game on full, i guess its something to do with directx 9c, maybe with directx 10 i can run it on full? anyhelp? or my specs are really low.