AnalogX PCalc

AnalogX PCalc allows you to enter any equation in the exact same format as C/C++, and it will process it with the same precedence, giving you the results in a variety of formats.

PCalc also supports the use of user-defined variables, and also has most of the common constants used already defined.
PCalc works on all versions of Windows, from Window 95 to Windows 7 and everything inbetween (including XP, Vista, Win2k, etc).