The thermistor constants a, b, and c are supplied by the thermistor manufacturer. So, you should be able to look at the manual or literature for the thermistor to get those constants and calculate the Rt and the temperature. I've linked to a document below that goes through an example calculation.
How Do I Calculate a Thermocouple Reading, That Uses Cold Junction Compensation, By "Hand?" http://digital.ni.com/public.nsf/websearch/DBF08A40F88F9C8D8525643A00578E53?OpenDocument
Also, if you have LabVIEW, there is a VI that does this conversion from voltage to temperature for you. The VI is called 'Convert Thermistor Reading VI' and can be found in the Data Acquisition>>Signal Conditioning subpalette.