Finding a Point on a Line

I would like your help on a math/GDI+ problem.  I want to find a point on a line, given the starting point on a line and a distance in pixels.  I'm using C# on Windows Forms.

Given a line from x0,y0 to x1,y1 and a starting point on that line of x2,y2 I would like to know the point value a certain number of pixels away.

So...

private PointF GetPointOnLine(RectangleF rc, PointF start, float distance)

Where rc is the bounding rect of the line, start is a point on that line, and distance is the number of pixels along the line from the starting point.