The following products are known to prevent Windows from recognizing Windows Mobile/CE devices (e.g. HC1, WDT60, and others) in Windows Mobile Device Center:
McAfee Security Scan Plus
The only known workaround is to uninstall the product(s) from the PC, then reboot. Simply disabling the product will not fix the problem.
Note: When uninstalling LiveSafe, if you get a message that says "Navigation to the webpage was cancelled", you will need to download a removal utility from McAfee and use that to uninstall LiveSafe: