In my HTML page, I need to check if Adobe Flash player is installed. If not, I want to automatically jump to another HTML page to tell the user that Flash player is required
window.location.href = "http://stackoverflow.com";
For local files this should work if you know the relative path: (In your case this works.)
window.location.href = "someOtherFile.html";
Maybe you could also do it absolute using this: (Not tested.)
window.location.pathname = "/path/to/another/file.html/";
The problem are the security measures of the browser vendors. Google has some good information about it.