These ones are tricky. But the following instructions are worth a try.
First do a complete uninstall of iTunes and related components (but don't reinstall just yet), as per the following document:
Removing and reinstalling iTunes, QuickTime, and other software components for Windows Vista or Windows 7
a. Launch regedit (Start >> Search Programs and Files >> type Regedit and open the regedit that comes up.)
b. Access the following Location: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData \S-1-5-18\Components
c. Perform the following actions
i. Right Click on Components Folder
ii. Select Permissions
iii. Select Full Control and Read Permissions options
iv. Select Advanced Button
v. Select Owner Tab
vi. Select to Change Owner to the Administrators
vii. Check the option “Replace owner on subcontainers and objects”
viii. Select OK when prompted that it may not change all.
ix. Select Apply
x. Select Apply again
xi. Exit the Registry Editor
Now try reinstalling iTunes again. Does it go in properly this time?