See section 5 of TS1538: iOS: Device not recognized in iTunes for Windows.
If that doesn't help it may pay to tear down and reinstall iTunes in the following manner...
Go to Control Panel > Add or Remove Programs (Win XP) or Programs and Features (later)
Remove all of these items in the following order:
- iTunes
- Apple Software Update
- Apple Mobile Device Support (if this won't uninstall press on)
- Bonjour
- Apple Application Support
Reboot, download iTunes, then reinstall, either using an account with administrative rights, or right-clicking the downloaded installer and selecting Run as Administrator.
See also HT1925: Removing and Reinstalling iTunes for Windows XP or HT1923: Removing and reinstalling iTunes for Windows Vista, Windows 7, or Windows 8. Some users may need to follow all the steps in the appropriate support document which includes some additional manual file and folder deletions not mentioned above.
tt2