software, you are sadly mistaken. You need a circuit in between that handles A-D conversion, and the USB protocol.
Make it easy on yourself, get a Phidget.
If you really insist on going this route, get a USB/serial board and hook it up to a PIC to manage the A-D.
USB/serial board : http://cgi.ebay.co.uk/USB-Serial-int...QQcmdZViewItem