windows 7 - How to convert two's complement? Calculator program and usage example

Possible Duplicate:
What super-calculator do you use?

I need a calculator program that runs on Windows 7 that can do two's complement conversions. Specifically convert a two's complement to regular decimal or binary. I'm told that the Win 7 calc does this.

Can anyone give a clear, specific example of how to do this on the built-in calc (or any other freely available program)? Thanks!