Shure | X2u - XLR to USB Microphone Signal Adapter | X2U | B&H
You could try something like this. Already $99 by itself and doesn't include the mic, but it has a monitor wheel where you can mix the output signal from the computer with the sound straight off the mic so you can monitor yourself with no latency.
Tried it with a mac, it's plug and play there, works and sounds good. Tried it with a windows xp machine too, but couldn't get it to work right. Got some sound but it was all scratchy and really low levels, so I wouldn't recommend it for a PC, but maybe I just have to install drivers (but it said I shouldn't have to..?)