Actually, windows does understand '/'. Even DOS did, it's only the command shells (command.com, cmd.exe, progman, explorer, etc) that don't.
If I understand the original question correctly, it may be a path problem. If 'include' is under the webserver document root, and we're talking about a file in the root, you should be asking for 'login/checklogin.php' i.e. without the ../