Same problem and aside from spending about 2 hours uninstalling and reinstalling until it works. I haven't found a real solution. By the way, even when I do get it working a BSOD usally messes it up again.
I don't believe I have AC97, how would you tell? My default XP installation has Realtek's High Definition Audio Codecs. So I've been trying those and I have them upgraded to the latest which was just released a few days ago. Still crackles.
Edit: I found my problem (stupid me). It was the program "Launch Manager" that I was using to make my hotkeys work on my laptop. I did a Windows 7 reinstall to see if that would fix the problem and kept checking the sound every time I added a new piece of software, that's how I found out what was causing it. When I have time I'll try out other versions of Launch Manager cause the one I'm using now doesn't control the volume, which is my main reason for getting it.