Downloaded GeForce 6800GT AGP BIOS 5.40.02.23.06 from:
and the NvFlash 5.50 utility from:
and flashed under FreeDOS (via boot cd, safe mode option) using command:
C:\> NVFLASH -4 -5 -6 BIOSFILE.ROM
I rebooted the computer to WinXP and made it enter sleep mode (S3) and come back from sleep until I got bored.
Sometimes one has to find the way by oneself...