If you are using .NET 4.0 you can leverage the Chart class found in the System.Web.DataVisualization assembly (you will also need to add System.Web). E.G:
var chart = new System.Web.UI.DataVisualization.Charting.Chart();
double result = chart.DataManipulator.Statistics.InverseTDistribution(.05, 15);
//2.131449546
Documentation: