uh... The microphone should plug in to a different jack than the line-out from your guitar and amp. Open the Record mixer (open Volume Control and select Options, Properties, Recording) and check the Select box for your microphone instead of the Line In box.
If you jammed the axe wire in the microphone jack, you'll have to fiddle with the volume setting and Advanced Controls properties to get your microphone settings back to where they're supposed to be.