This installer does not work on 64-bit Windows versions. It will try to install in c:\Windows\system32 but instead the OS redirects the installation to c:\Windows\SysWOW64. Paradoxically system32 is only accessible to 64-bit programs. 32-bit programs, such as Morgan's installer, are redirected to SysWOW64.
The installer will seem to work initially because when it sets MA as default screensaver it uses MA 3.1 which it installed in c:\Windows\SysWOW64. However, when you set another screensaver as default and then try to pick MA again it will revert back to the old 3.0 version which is still in c:\Windows\system32.
The start menu shortcut also points to this old 3.0 version.
A workaround for 32-bit installers on 64-bit Windows is to install in c:\Windows\Sysnative\. This will redirect the installation to the true c:\Windows\system32\ folder. Leave it to Microsoft to think up something as crazy as this.
My advice for 64-bit Windows users is to wait for the SereneScreen installer.
If you already used this installer on 64-bit Windows you need to move c:\Windows\SysWOW64\MarineAquarium3.scr to c:\Windows\system32\MarineAquarium3.scr and overwrite the old version. (use Windows Explorer to do this because if you use a 3rd party file manager which happens to be a 32-bit application it will run into the same redirection problems as the installer)