Problem with mic input and speakers

Hey, sorry if I'm dropping this in the wrong section, but I need some help here. I'm the kind of guy who prefers windows as an OS (I just can't multitask as well on OSX) so I'm mostly running XP Pro over boot camp. Now, I can't really give specifics as to this comp, since it's my dad's, but it's about 2 years old.
Here's my problem: Anytime I try to run an input through the mic jack, whether it's a mic or an instrument or whatever, it just comes out the internal speakers. I have no idea why it does this or how to fix it.
Any ideas?

Do you have external speakers attached to the machine? Are you using the correct input jack and output jack? The headphone jack on most machines acts as the audio output - that's where you would plug the speaker set into.

Regards.

I've got external speakers hooked up through an amp, out of the headphone jack. Anything connected to the mic input just comes out of the internal speakers very compressed, with only the loudest bits registering.