If your PC has Windows XP, you should have
ActiveSync installed and running, in order to have a WM smartphone synchronized. Otherwise, if it has Vista or 7,
WMDC should be used instead.
If you already happen to have the correct sync program on your PC, and yet, are unable to sync your device, firstly, reboot both your PC and device. If you keep unable to sync, try to remove the partnership between your PC and your device on both of them, and try to connect your device again. If the failure persists, try to uninstall and reinstall ActiveSync (or WMDC), reboot your PC and try again.
Hope you are able to sort your problems out.
Cheers!
Carlos