The built-in calculator for Windows 7 is pretty good (and you can use it on Vista as well). If you fancy a more Apple-like experience, iCalcy replicates the built-in iPhone calculator in a standalone application.
Looks aside, I couldn't spot any functions on iCalcy that aren't already in the Windows calculator (indeed, it's currently missing some of the scientific options such as trigonometric calculations). However, if you want to make your Windows experience more Apple-like, this is a simple way of doing it. iCalcy is a free download for Windows only.