Now we're almost getting somewhere.
Can you check to see if Photoshop CS5.1 is actually updated to Photoshop CS5.1 12.1 please? DO NOT rely on the Adobe auto updater to tell you there are no updates availble. Look at "About Photoshop".
Also important is how much available disk space you have on your designated scratch disks and on the the disk to which you are trying to save?