Irritated iPhone users are beginning to report major difficulties in getting their iPhone to sync with Windows 7 systems. Particularly 64-bit Windows 7 systems, based around the P55 chipset. The iPhone is usually (though not always) recognised, but iTunes then complains that it can’t connect to the unit because of an “unknown error”, usually (though again, not always) 0xE8000065.
Disabling USB power management appears to be one solution. Click Start, type DEVMGMT.MSC and press [Enter] to launch Device Manager, then click View > Devices By Type. Expand the Universal Serial Bus controllers section of the tree, right-click each USB Root Hub entry in turn, select Properties > Power Management, and clear “Allow the computer to turn off this device to save power”.
Reboot your PC after this tweak and try again. This works for some, but if you’re out of luck then check the Apple Discussions thread for other ideas.