Strange problem with recording I haven't heard of before

[cucumberian]

Okay, whenever I try to record anything off of my computer's sound card by pressing the big red Record button, strange stuff happens. Only the left stereo track gets recorded, an out-of-sync duplicate of the sound being recorded starts to play over the original sound while Audacity is in the process of recording, and worst of all; this aforementioned duplicate gets louder and louder as recording goes on until the red and green bars in the upper right are well over maxed out. This effect is present in what Audacity puts out as the final recorded track.

My computer's volume level is properly set (I think), and I am not touching anything within or outside of Audacity as recording happens.

Anyone know what might be causing this? I am running Windows XP, if that helps.

[cucumberian]

Okay, I uninstalled and reinstalled Audacity and the volume problem seems to have gone away (EDIT: it seems to show up at random), but I'm getting this really weird echo effect in all my recordings. I think this results from that out-of-sync duplicate also playing thru the sound card while I record. So how can I stop Audacity from playing the sound it's recording, while it's recording?

Oh, I'm not entirely sure whether I should record the sound card with the Wave Out Mix, Line In, or Aux setting.