Windows and UNIX websites can be accessed internationally by everybody. The main difference exists in the software provided to the host servers data. Windows most always provides the best software and services in all of its purposes. It is a very reliable operating system and its programs are effective and functional but UNIX remains a little ahead in these areas due to increased stability and the ability to run long periods of time without rebooting. At the same time, in the modern world Windows meets the needs of its users better than UNIX. Wherever there are Microsoft products such as MS Access databases, ASP for delivering content, and VB Script site enhancements you will need to create your site with Windows. UNIX is not compatible with those systems. If your web host uses them you must comply and use Windows even if you normally do not. The things that UNIX provides can be found already available in the Windows environment.
Windows supports a wider range of development languages that web sites are written in, but unless you're site is written specifically with ASP or ASP.NET then Unix is as capable as Windows. Windows has been a reliable OS since Windows 2000. 2003 was a great operating system but Windows 2008 is brilliant. Windows rarely requires a reboot on updates (it does sometimes) but in terms of reliability there's nothing in it between Windows and Unix. I've worked for a Windows host since 1999, and I know very little about administering Linux, but I can say that apart from ASP and ASP.NET development languages Linux is every bit as capable. Each OS has their strengths and weaknesses, but as general hosting platforms it largely comes down to personal choice. I prefer windows, but that's because my background (and everyone who works with us) has been in Windows development and we understand the system intimately. We can set it up for security, maintain it, troubleshoot it, and keep it working. I'd have not problem using Linux except for the fact that we don't have the skills to administer it properly. That is complete nonsense. If you develop the web site using ASP or ASP.NET then you will need to use a Windows host. These languages have been ported to operate on some Linux platforms but they aren't as fully featured or robust. Apart from that, it doesn't matter what operating system your host uses or what you use to develop your web site. Sites developed in PHP on a Linux platform will work in Windows and vice-versa. Although all of our systems operate on Windows, we use ASP and ASP.NET to run our control panels, and MS SQL for our databases, clients can move their PHP app and MySQL database to our hosting and it will work just fine - and if they leave to go to a Linux host instead, it will work there too.