I seem to remember some problems of that kind on both ATI and Geforce cards. I think the game uses a way of writing to the screen buffer so that the driver doesn't have access it for anti-aliasing. That technique was probably faster on some old hardware. My guess is that the windows XP drivers do not allow that kind of rendering so Half-Life uses a more "standard" method. Here are some things you can try:
Upgrade to a newer version of Half-Life
Using 32-bit color (put -bpp 32 in the command line)
Look in the ATI drivers for something related to buffer flipping (probably not there anymore in driver from 2005).
You can also try to change gl_d3dflip to 1 in the console followed by vid_restart.
Hope any of this helps.