New Features in Windows 7’s Calculator
Windows has always had a calculator, but for the most part it was a pretty cheap calculator. Windows 7 has substantially improved the calculator, adding more functions. You can now use the calculator in Windows 7 for everything from calculating your recent gas mileage to statistical analysis.
To run the Calculator, choose Start→All Programs→Accessories→Calculator. You’ll probably see the standard, basic Calculator window.
With the Calculator in standard mode and with the basic configuration, you can do simple mathematical operations by entering the values to be computed separated by the operators (+ for addition, – for subtraction, * for multiplication, and / for division) and then clicking the equal (=) key.
Keep in mind that you can enter your numbers and mathematical operators into the Calculator by either clicking the onscreen buttons or using the number keys or numeric keypad on your keyboard.
The Calculator has three separate modes that you can switch to by selecting them from the View menu (or pressing the shortcut keys shown in parentheses):

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

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 (and if you don’t know what that is, you probably don’t need this mode).

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

Date Calculation (Ctrl+E): Adds fields for performing calculations between two dates.

Worksheets: Adds fields for calculating results, such as finding the necessary down payment or your monthly mortgage payment; the residual value of a leased vehicle; or the fuel economy of your vehicle.
Press Alt+1 to return the Calculator immediately to standard mode from any of the other three modes and press Ctrl+F4 to return to the basic configuration from any of the other available configurations.