When installing or updating iTunes for Windows on a 64-bit Windows Vista or Windows 7 system, you may see a "The Windows Installer Service could not be accessed" message, as per the following screenshot:
Other 64-bit installers also do not install, but 32-bit installers do install on the system.
A registry key value may have been set on your system which disables 64-bit installations.
Note: The following instructions involve a registry edit. Back up your registry prior to proceeding.
(1) Launch Regedit as an administrator. In your Start Search field, type regedit. Right-click on the Regedit entry that comes up and select "Run as administrator" as per the following screenshot:
(2) In the left-hand pane of Regedit, click the triangle next to HKEY_LOCAL_MACHINE to expand its contents.
(3) Expand the contents of SYSTEM
(4) Expand the contents of CurrentControlSet
(5) Expand the contents of Services
(6) Select MSIServer
(7) In the right-hand pane of Regedit check for the presence of a WOW64 REG_DWORD 0x00000001 (1) value as per the following screenshot:
(8) If that value is present, right-click on WOW64 and select "Modify", as per the following screenshot:
(9) In the "Edit DWORD (32-bit) Value" screen that comes up, change the 1 in the "Value data" field to 0 (zero) as per the following screenshot:
(10) Click OK. The WOW64 REG_DWORD value should now be 0x00000000 (0), as per the following screenshot:
(11) Quit Regedit.
(12) Restart the PC.
Now try installing or updating iTunes for Windows again.