Hi, thanks for getting back to me.
I opened my Utilities > Activity Monitor
Found the SafariNotificationAgent and tried to quit this first, didn't shut it down... so I then force quit it.
I then tried to install but this still came up with the error.
But I then tried uninstalling Flash and then reinstalling Flash and Acrobat and this worked a treat... it then installed fine.
Please note: when I uninstalled Flash I went into:
Applications > Utilities > Adobe Installers > Adobe Creative Suite 5.5 Design Premium
It was here I uninstalled just Flash so it took all the parts out.