@Cyrus: Since Windows 7 came out there is now some native support for AVCHD in Windows, but you need to have either Windows 7 Home Premium, Professional, Enterprise or Ultimate in order to access it.

You need to download Windows Live Movie Maker in order to do any editing with it but you should be able to preview AVCHD footage within Windows Media Player, as long as you have one of the previously mentioned versions of Windows 7.

Hope that helps!