Using OpenGL In Newer Games

I have been puzzled. This is my PC

1.4ghz AMD Atholon
256mb DDR ram
GeForce 2 GTS 32mb
Windows XP

I installed the Medal Of Honor demo on my computer when I had windows 98, it worked perfectly with all detail options on full at 1280x1024 resolution. I upgraded to XP and all of my games performance went down the tube. MOHAA now only runs at 640x480 with medium detailing options. I want to know what happened.

How can I switch to the OpenGL drivers for the game. It brought me to a opengl download but when I went to install it the program told me it only worked with Win95 and Win98. I read around and it seems that the games I can still run at high resolutions with high detail options (Half-Life, Hitman) all used the OpenGL drivers, not Direct3d. I want to know if my gamin performance will improve if I switch to OpenGL drivers. How do I do that with MOHAA (full version) and or Unreal 2003 (full version). Thanks!