Ok, first you need to understand what each does (if you don't already).
Mic preamps amplify the very small microphone signal to a more robust and common voltage called line level. Audio circuits have a linear portion of their transfer function, and defining a standard line level helps to get all the gear to work together.
Now an interface does something entirely different. It takes a line level signal and converts it to a digital representation of that signal, called A/D conversion. It also performs the reverse operation, D/A conversion, to convert the bits back to a voltage waveform.
Now back to your question. A mic preamp cannot be an interface, though some mic pres have an interface built in, following the preamp circuit. They then have a digital output, like S/PDIF, ADAT, TDIFF, etc., and would need a similar digital interface in the computer to hook up to. If you plug the output of a mic pre into a 1/8" input on a computer, you're plugging into a built in soundcard, or most commonly a soundchip. If it's a line level input you're ok, though built in soundchips usually don't provide the best sounding interface. If it's a mic level input, you're going to get a lot of distortion because you already amplified the signal and it's now too hot for the mic input on the computer.
-RD
