I want to purchase a Mike to use for recording instruments on my computer and I have no Idea what kind to get. I'm looking to spend somewhere betwen 50 and 100 dollars. It's mostly to record acoustic guitar, Mandolin and to mic my guitar amp. If someone one could point me in the right direction it would be appreciated.

i see the shure sm57 mentioned a lot as a standard workhorse microphone. a friend of mine does a bunch of home recording with one of them that sounds quite good. there's technical info here and a glowing review here. they go for around 75 bucks on ebay.
however (as i've been finding out recently) the mic inputs on most computers are absolute shit, meaning you would also need a preamp if you want it to sound decent. ART makes some of the cheapest commercial preamps--you can pick one up for about 40 bucks. i haven't heard any of them, but from the reviews i've read they're frowned upon by audiophiles.
i would recommend a subscription to tape op magazine. it's free, full of tons of useful info and interviews with lots of the bands discussed frequently discussed on obner.

*has a Shure SM57*

Then again, I'm not the most technical person in the world, which is probably why I have a MXL 990 Vocal mic that I haven't even used yet (phantom power? Buh? )

phantom mics usually have a battery input too... just sayin. Hook that shit up.

if you're only gonna buy one cheap mic, it's the sm57.

hey paranoid android - go here: http://www.obner.org/bb/viewtopic.php?t=15435