Realtek ALC889A line-in problem

My mobo is a Gigabyte MA790GP-DS4H with an AMD Phenom 9950 Black. I've been running Windows 7 Beta and RC (64-bit) on it. I never had any problem with the sound out (listening on a pair of Tritton 5.1 headphones). No problems. But one day I plugged in a line from my bass amp to the line-in on my Realtek onboard sound. It seemed to work fine. I got clean sound out. But I had to update something so I restarted my computer just a few hours later. Ever since then, I hear nothing on the line-in except a continuous "pop, pop, pop". Not one long pop, not hissing, just a recurring pop. It's not even very loud. But I can't hear anything else on that line-in. It's not the amp because I had used it with my microphone as well and it worked fine, and now it also won't work with the microphone. The Creative X-Fi card I got is handling the line-in from the amp or microphone just fine. The Realtek sound out still works just fine. Anybody got any ideas why the line-in doesn't work? I've got the latest drivers, but remember that it initially worked just fine. Is this a hardware failure?

Thanks for any input.

Thanks for the suggestion, but that was the first thing I tried. I suppose I should have said what I've done so far. First I did a system restore, but that didn't work. Then I removed and reloaded the drivers. That didn't work. I haven't tried re-installing Windows, but I'd hate to lose what I've got now. I eventually just got a new sound card, a Creative X-Fi that works just fine.