If your PC is on a domain (typically a work network) the administrators may have the following MS virus alert KB http://support.microsoft.com/kb/962007 which stops WMDC dead and windows mobile devices do not install when connected with USB.
The trick to fix this is, open Regedit on PC.
Right click and select permissions on HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion
Select "Owner: TrustedInstaller" Change TrustedInstaller permissions to: Full Control & Read, not inherited
Next, right click and select permissions on HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\SvcHost
Select Owner: System
Change TrustedInstaller permissions to: Full Control & Read, inherited from CurrentVersion
(Actually in the end I changed all permissions in these 2 to Full Control & Read)
Save your work. Reboot and try to connect your windows mobile device via USB and with any luck WMDC will download (again, even if it installed) and the device will connect.
Full article here: http://forum.xda-developers.com/showthread.php?t=1304483 thanks to Whatdoyaknow for posting a solution.