Following my original feedback- I have discovered the solution - If you are unable to upgrade to version 5.1.3 using automatig updater tool - do foolowing: go to Apple website --Mac OSX--Downloads--select Apple downloads--Final Cut Pro 5.1.3--log in under user name and enter the serial number of your software -- download 5.1.3 upgrade file which DOES INSTALL without any problems.
Apple must have posted damaged or faulty file on server which is accessed by automatic Software Update tool.