fx-Calc

x-Calc is an advanced desktop calculator for Windows. The biggest advantage is the simple and intuitive handling of complex mathematical tasks.

fx-Calc lets you define, analyse, visualise, and calculate scientific functions and is a full-featured replacement and/or enhancement of the old fashioned Windows calculator. In addition to calculation and visualization, you can analyze and solve scientific functions and perform linear regression.

Even if the window concept may not look advanced, it provides the big advantage of opening similar windows multiple times to compare the results as needed.