I have been having the same prob with FS2000 - irregular clicking sound, and double sound when I switch between desktop and full-screen mode; and frame rates suddenly dropping to 1.3/sec or something ridiculous at every critical moment. The following steps (as recommended above) seem to have worked:
1. Download latest nVidia driver
2. Download Directx 9.0b
3. Uninstall AVG 6.0 (there doesn't seem to be any way of disabling it) before installing nVidia driver and Directx
4. Disable onboard sound using the jumpers on the motherboard
(My motherboard (ASUS CUV8X) has an onboard AC97 audio codec, and I also have a Soundblaster Live! 5.1; they were conflicting)
5. Reinstall AVG 6.0
Strangely, a few clicks still remain in the sound, but the graphics seem steady at around 13 frames/sec - hardly fast (but then my PC is old), but certainly more playable.
Thankyou
xMymblex
