Jaime,
There are a few examples on PID Control loops in NI Developer Zone. One that doesn't require hardware and sort of lets you explore the theory behind it (chaning the inputs and seeing the results) can be found at:
http://zone.ni.com/devzone/devzoneweb.nsf/opendoc?openagent&9906333B078AFAC286256870004FCB11&cat=40F54238B22CB4CA862567E6006F5FE0
If you search on PID from the mian page (ni.com/zone) you should be able to find some other examples that interface with hardware as well.
Kamran