Windows 7 have a new , much more advanced calculator than in previous windows versions.
To open the calculator (Start>All Programs>Accessories>Calculator
Or you can run it by typing “calc” in search.

Keep in mind that you can enter your numbers and mathematical operators into
the Windows Calculator directly from the keyboard using its number keys or
numeric keypad rather than always clicking the numbers and operation symbol
buttons in the Calculator window.

Windows Change Calculator modes:

Statistics (Alt+1):Basic mode

Windows 7 Basic Calculator

Scientific (Alt+2): Adds keys for performing trigonometric calculations —
great for all the engineers out there.
Windows 7 Scienfitic Calculator

Programmer (Alt+3): Adds keys for performing programming calculations
in different number systems, including hexadecimal, octal, and binary number
systems, along with the usual decimal system using different word and byte
sizes.

Windows 7 Programmer Calculator

Statistics (Alt+4): Adds keys for doing statistical calculations involving
linear regression.

Windows 7 Statistics Calculator

In addition to the basic configuration — perfect for doing straightforward
computations — the Calculator also offers the following specialized configurations
(also available from the View menu) designed for more complex
calculations:

Unit Conversion (Ctrl+U): Adds fields for converting all sorts of measurements
(angles, pressures, temperatures, weights, time, velocities, mass,
and so on) from one set of units to another.

Windows 7 Unit Conversion Calculator
Date Calculation (Ctrl+E): Adds fields for performing calculations between
two dates.

Windows 7 Date Calculations

Ctrl+F4 to return to the basic configuration
from any of the other available configurations.

Windows 7 trial version can be downloaded from here