As you know Windows 7 contains a new redesigned Calculator which has new GUI and new options. You can get more information about it in following topic:
If you can't use Windows 7 but like the new Calculator, then you can get the same calculator in Windows XP and Vista.
Our friend "grdcityboy" @ DA has created a small application which looks very similar to Windows 7 Calculator:
Its fully functional but only "Standard" mode works because of limitations.
You can download it using following link:
NOTE: You'll need to install Microsoft .NET Framework to be able to use this app.