It sounds as though the thermistor is the lower resistor (the one connected to ground) in a potential divider. If so, the resistance of the thermistor relative to the fixed resistor is equal to a/(1024 - a), where a is the analog reading, assuming the potential divider is being fed from +5V and you are using the default 5V analog reference. You can then calculate the temperature using the Steinhart-Hart equation, see http://www.daycounter.com/Calculators/Steinhart-Hart-Thermistor-Calculator.phtml.
Temperature sensors that give an output voltage linear in temperature (e.g. LM34, LM35, LM335), or that give a digital output (e.g. DS1820B) are easier to work with.