The problem is probably your application, not your installation of Windows.
As far as I'm aware, Windows (even English) installs with Greek characters in its fonts by default.
You need to ensure that all files in your application are in a consistent encoding, and that that is the same encoding as the browser thinks they're in, and that all data operations also respect that particular encoding for storage, retrieval and integration with third party systems.
This will involve your editor, database, email and other things being encoding-aware - consult specific documentation for more information.
Mark