You must have not practiced the 'Safe Mac Update' rule ...
01. Before installing any Apple update(s) - launch 'Disk Utility' ('/Applications/Utilities/' folder) and perform a 'Repair Disk Permissions', a minimum of once. Some prefer booting from the Installation CD / DVD and performing a 'Disk Utility' 'Repair Disk', and then a 'Repair Disk Permissions' (or two), on the boot hard disk drive.
02. Install the Update(s).
03. After installing any Apple update(s) - launch 'Disk Utility' and perform a 'Repair Disk Permissions' a minimum of once.
And yes, sometimes - moving to the 'Desktop', or deleting, the respective Application's preferences file(s) ('com.apple.iTunesHelper.plist', 'com.apple.iTunes.eq.plist', and 'com.apple.iTunes.plist - for 'iTunes')'; or, rename them (say by appending '_' to them), does solve a - failed to launch iTunes' problem.