You can use your sound card for analog inputs and outputs, but the bandwidth and feature set is fairly limited. Look at the sound input and output VIs.
You can access the USB ports using the VISA API, but USB is a serial protocol and does not have many pins. I would suggest you get something like the USB-6008. If you are in an academic environment, contact your local National Instruments sales rep for a possible discount. If you want to go really inexpensive, you can try a tethered Arduino.
