A number of troubleshooting techniques are available to deal with an interrupted iTunes installation. The most likely causes for the problem are a previous installation of the software that hasn't been removed properly or a corrupted installation file. To begin with, make sure you are using the most recent iTunes installation package for your operating system, downloaded from the official Apple website at apple.com.
VBScript Error or Error 2738
If the installation is interrupted by a message referring to VBScript or error code 2738 this points to a problem with missing or corrupt Visual Basic Script files. If you are using Windows 2000 or XP, download the latest VBScript package from the Microsoft website; if you are using Vista or Windows 7, the necessary files should already be installed. Launch a command prompt window from an administrator account and enter "regsvr32 vbscript.dll" to re-register the necessary files with the operating system, then try the iTunes installation process again.
Installation Stops at "Registering Modules" Stage
If the installation hangs at the "registering modules" stage of the process, it's likely that an anti-spyware program is interfering with the process. One package known to cause this error is SpyCatcher. To attempt to fix the issue, disable your anti-spyware software before launching the installation file. To protect your PC, disconnect it from the Web while the anti-spyware software is disabled -- the iTunes installation package does not need Internet access to complete the program setup.
"Could Not Open Key" Alert
If an alert stating that iTunes "Could Not Open Key" appears, this suggests you are trying to install the software in a Windows user account without administrator privileges. Try signing in using an administrator account and attempt the installation again. It is also possible that you are attempting to upgrade the software from a different user account to the one used to install iTunes in the first place -- switch back to the original account to see if the problem is fixed. If the problem persists, there may be an issue with the Windows registry -- run a system restore from Control Panel to roll back the registry settings or use a third-party registry fixer (such as Registry Repair from glarysoft.com) to try and troubleshoot the problem.
A variety of different issues can affect and interrupt the installation of iTunes. As general rules, make sure you have applied all of the latest Windows updates, and completely removed any previous iTunes installations before proceeding. You might also have to clear any temporary files associated with your Windows user account. Full instructions for how to do this, as well as troubleshooting tips for other installation errors and messages, are available from the iTunes section of the official Apple Support website at support.apple.com.