How to Delete the Installer Cache on iTunes
When you install Apple's iTunes software on your computer, it creates a folder called Installer Cache for troubleshooting purposes. If you experience a problem with iTunes, you can repair it using the original installation data stored in the cache. However, the Installer Cache folder may also store the installation data for previous versions of iTunes that you no longer use. You can safely delete these older versions to reclaim hard drive space.
Open the "Start" menu, and click the "Computer" icon.
Double-click the "C:" hard drive icon.
Click the "Organize" button in the upper left corner of the window, and then click "Folder and Search Options."
Click the "View" tab.
Click the "Show hidden files, folders, and drives" radio button, and then click "OK." You should now see a folder called "ProgramData" in the Windows Explorer window.
Double-click the "ProgramData" folder.
Double-click the "Apple Computer" folder.
Double-click the "Installer Cache" folder. If you have upgraded iTunes on your computer multiple times, you may see several folders with the name "iTunes" and a version number. For example, one folder might have the name "iTunes 10.0.0.68."
Right-click each of the iTunes folders except the one with the highest version number, and select "Delete." Click "Yes" to confirm.
Launch "iTunes" and confirm that it works properly before emptying the cache folders from the Recycle Bin.