If you have USB connection problems with ActiveSync or if the synchronization is stuck on "Looking for changes", you should have a look to the following guide:
If you don't have the last version of ActiveSync, you may also try to install ActiveSync 4.5
Note that Kerio firewall seems to have compatibility problems with ActiveSync. Sometimes the mobile never succeed to connect or the process rapimgr.exe can use 100% of the CPU. Disabling the firewall is not enough to solve the problems and you have to uninstall it. It is recommended to use ZoneAlarm instead.
Another possibility is to change the way your mobile device is connected to the PC. On recent models (some particular versions of Windows Mobile 5.0 and on 6.0), it's possible to change how your mobile device is connected to your PC with USB. You can go in "Start/Settings/Connections/USB to PC", then uncheck "Enable advanced network functionality". That will switch from a "RNDIS" to a serial USB connection and your mobile device will connect to the PC as Windows Mobile 2003 devices did.
If you don't have this option on your Windows Mobile 5.0 device, you may try to use a tool called usbswitch.
Another solution is to use Bluetooth connection. It works very well and seems to be more reliable than USB connections.