If you are having trouble getting your mobile device to connect to your PC with a USB connection, this information may be helpful.  These tips can apply to ActiveSync on Windows XP, or Windows Mobile Device Center on Windows Vista or Windows 7 (or higher).

Why isn't Windows Mobile connecting to my PC?
Note that Windows Live Once Care can interfere with the USB connection to your PC.

ActiveSync USB connection troubleshooting guide
This topic also applies to ActiveSync 4.5