What WM device you are using?
Have you try changing the connection setting on ur pc sync4.x?
usb is diff from the other.
I am facin one problem that I have one Windows Mobile 5 device, this device working fine with USB connection with Active Sync 4.5. but when I am using ethernet cradle my device is taking an IP but not connecting with Active Sync 4.5.
The previous version of Active Sync 3.8 is working fine with ethernet cradle and my device becomes connected without any trouble with my PC through that old Active Sync.
So any one have this idea. how to resolve.