Ok, I'm having a serious problems recording audio on my PC. :(

I do my recording through my PC's sound card - the onboard Realtek AC97.

Everything was fine until a couple of weeks ago - when something odd happened to any track I tried to record. I started recording a guitar overdub on my PC for a multitrack session I'd recorded elsewhere.
As well as picking up my guitar through the line-in port, the recording software also recorded the drums and other tracks coming through the speakers!

I think it may be a hardware issue, I get this effect with Adobe Audition and Reaper! I don't understand why this problem has suddenly cropped up. I hope some of you guys can help me out. :?