Mic & Speaker Troubles

I got a new computer not too long ago (Windows XP), and so far, it's been
great. However, it's got some sound issues:

The main problem I've been having is with my mic. I use an online chat
program called Ventrilo when playing online games. What friends have told me
is that when I speak in my mic, any sound playing on my computer (winamp
songs, game sound effects, etc.) records as well (and very loudly at that, I
might add). This is the same mic I used with my old computer (also Windows
XP), and it never gave me any trouble, so I'm sure it's something with this
computer. I'd really appreciate any help I could get with this first problem.

The second isn't as bothersome, but I'd still like to get it resolved. While
sound works perfectly through headphones, it does not play through my monitor
speakers. The headphones work well when plugged into the computer directly,
but when I tried plugging them into the monitor's headphone jack, they also
didn't play. I was wondering if maybe it has something to do with the fact
that this monitor originally did not come with my computer as a package- it's
a flatscreen HP monitor, while my computer is Compaq. Could that possibly be
the problem, or something else...?

Any help is much appreciated! Thanks!