1. Did you make any changes to the computer before the issue occurred?
I would suggest you to follow the steps from the link below and check if it helps.
Method 1: How to restart the Apple Mobile Device Service (AMDS) on Windows:http://support.apple.com/kb/TS1567
Method 2: If the issue persists then set your computer in Clean Boot state and check if the same issue occurs.
By setting your system in clean boot state helps in identifying if any third party applications or startup items are causing the issue.
How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows 7:
Note: After clean boot troubleshooting step, follow Step 7 in the provided link to return your computer toNormal startup mode.
Removing and reinstalling iTunes, QuickTime, and other software components for Windows Vista or Windows 7
Hope this information is helpful.
Neil Patrick Harris(Zachary Knight)