When Upgrading iTunes to 10.5.0 on a Windows XP PC, the error message was: "There is a problem with this Windows Installer Package. A program run as part of the setup did not finish as expected. Contact your support personnel or package vendor.
I've tried uninstalling and reinstalling iTunes and Quicktime all without any success.
I've also tried a repair through Add/Remove programs - no success.
I have the latest Windows updates and I also installed Windows Installer Package 4.5.
The last message I got was: "The installer encountered errors before iTunes could be configured. Errors occurred during installation. Your system has not been modified. Please run the installer again or click Finish to exit."