“iTunes +QuickTime” has an invalid signature. It will not be installed.
I recently ran into this issue when running Apple Software Update on my Windows XP machine and searching the Internet did not give me any good leads. What I did seems to be working so I will share:
Go to Task Mangler (Task Manager for those who want literal names) and End the Processes anything to do with Apple.
Go to C:\Program Files\Common Files\ and remove the Apple folder.
Launch Apple Software Update and it should have the iTunes and Quicktime under the Optional software again.