Plugging a microphone into a guitar amp using a guitar input is a very bad bad idea.

This is because the voltage put out by the microphone is much too low for the input on the amp.

Mics put out Lo Z signal and Guitars put out Hi Z signal,

You need to plug the mic in to a Mic level (not line level) input on your sound card.

Or use a preamp.

Line level will also overload the guitar amps input.

My guess is that the computer is not set up properly.