Open-source Windows alternative
ReactOS is a free and open-source, Windows-compatible OS that looks similar to Windows XP.
Designed to be a Windows replacement that behaves just like Microsoft's flagship OS, the operating system has just been updated to lay the groundwork for running Windows Vista/8/10 software.
Read more about what ReactOS can do, and check out this visual guide to what ReactOS looks like and the apps it can run.
Here you can see balloon notifications on the ReactOS desktop, a feature that has been fixed with the latest update. Be warned, however, that ReactOS is alpha software, so expect a lot of bugs and missing functionality.