Yes, you will want either a mixer or a standalone preamp. Mixers have built in preamp's.
The preamp brings(amplifies) the low voltage mic signal up to a suitable level for feeding the input of your sound card.
Why it doesn't work when plugged into your line in - I don't know, possible there just isn't enough gain because your not using a preamp, or the level is down in the windows sound control/mixer thingy.
I guess you have some sort of XLR to 1/8" adapter? - your losing a lot of your signal right here. Stay balanced.
Anyway, how many instruments do you need to record at one time? You will only be able to record 2 tracks at time, this is a limitation of your sound card.
But, if you plan on recording drums, and micing all the toms, kick, snare etc - a mixer can do this, providing it has enough inputs - 1 input/preamp per mic.
If, on the other hand you only plan on recording one instrument at a time, then maybe a single preamp unit will suit you. Art Tube MP, Bellari etc. products, won't recommmend any - Lots of info on this BBS. Do a search for "preamp".
You should also read the info on the main page:
http://www.homerecording.com/newbies.html
[This message has been edited by Emeric (edited 06-24-2000).]