As I understand it, thermistors need to be calibrated individually if you want accurate results, as the manufacturing tolerances and the nature of the devices makes it hard to produce precisely consistent results. You'd need to record the resistances at a variety of temperatures to make up a look-up table.
But they are cheap, though, which makes up for it 
In your Arduino code, how are you converting from the 0-1024 readings you get from the analog input pins into temperature?
Worth looking through this - skip down to the temperature conversion section...