I tried XP beta 2 with my GTXP and had similiar problems. I read on hercules' forums that it has to do with DirectX 8. WinXP comes with dx8 installed and the gtxp drivers arnt meant for dx8. if you read the faq on hercules page it mentions not installing dx8 on win9x/me/2k. aparently the drivers for the gtxp dont get along with dx8. i once installed win2k with dx8 and the hercules drivers and had all sorts of problems. now im using win2k, dx7 and the 2.01 drivers and have no problems. Also, ive had strange sound issues with mp3's and playing divx movies, the sound glitches out for a moment. This is due to the "digital in" being unmuted in the control panel. Aparently the port runs in polled mode and the driver is constantly polling to see if a stream is present, when it polls, the audio glitches. By muting the digital in when not in use these problems went away.
Hope this helps.
Ultrix