Here's one for 81 dollars that I've recommended to someone before and have worked with his vocal tracks recorded with it, and the results are great for such a cheap unit:
Alesis iO2 Express Audio Interface | Musician's Friend
It doesn't need to be Firewire unless you have a lot of channels. These 2-channel units run fine on USB.
Anyway, it's fairly simple to set up. Also has phantom power if you use a condenser.
You just have to find the right driver for your host program. If it uses ASIO, then you'll probably end up downloading the ASIO4ALL driver with this unit. No biggy though. It's free.
Since I know what the results are like on this unit, I can safely recommend it above others I haven't heard, from a mixing analysis standpoint. You just have to be mindful of your recording levels. For the price, you can't do much better.