PT100 is a PRT (platinum resistance thermometer) and is also known by the term RTD (Resistance Temperature Detector). I did a small interfacing project using the PT100 and 8051 when I was in undergrad school. As steve said you need to have a current source which will cause a voltage drop across the terminals of the RTD which you will be measuring using an ADC. You can calculate the resistance from the V and I values. This resistance is proportional to the temperature at which the RTD is kept. So you will have to implement a lookup table in your 8051. The resistance and temperature relationship is fairly linear. But that depends on your accuracy requirement. Will try to dig up some old notes which I might be having.
good luck
Ashish