Jammed after start up - login items load, but desktop is unresponsive

When I start up my iMac, everything seems to load up, start up items load, but the desktop is totally unresponsive. Apps will not run, windows will not open, close, resize or move, menu does not respond. All I have is a frozen screen, nothing responds except the mouse cursor.

Whilst the desktop is unresponsive, the Activity Monitor icon in the Dock shows sporadic activity and there is disk noise from the boot drive, indicating it is doing something.

I have tried:

- resetting the PRAM - no effect, unresponsive desktop as before

- restarting in Safe Mode & preventing start up items from loading - no effect, unresponsive desktop as before

- starting up from my backup clone external hard disk ( made using SuperDuper) - starts up to the backup disk's desktop, then jams as before

- starting up using the Recovery Partition, (10.8.2) created when I first installed 10.8 - jams on OSX Utilities screen, no options are selectable

- starting up using an install disc that I made on a USB stick (10.8.2) - jams on OSX Utilities screen, no options are selectable

The problem started soon after I updated Safari & iTunes to the most recent versions yesterday 23 October 2013