If Partnerships have previously been set up between a Windows Mobile/CE device and two different PC's (the maximum allowed), a New Partnership cannot be set up without deleting one of the existing partnerships.
1. To delete a partnership, click the ActiveSync 
icon in your Windows System Tray. The ActiveSync screen appears.
2.On the ActiveSync screen, select File > Delete Partnership. The Delete a Partnership screen appears.
3.On the Delete a Parntership screen, highlight a partnership and click the Delete button.
Special Note:
There is a unique situation that can sometimes be created by performing a Hard Reset on a Windows Mobile/CE device. A hard reset of the device does not always reset the device's registry. This makes it impossible to set up a new partnership, because the device's registry will still show that two partnerships exist, as shown above.
To manually reset your Windows Mobile/CE device's registry, perform these steps:
1.Go to My Computer (on the device)
2.Go to Utilities
3.Tap the BootMode icon
4.Tap Open from the menu
5.Hold down the Left Arrow key (under the ESC key) and tap Warm Boot button
6.Answer Yes when asked if you want to perform a cold boot.
It will hard reset the device.
When it finishes booting...
7.Go to My Computer
8.Go to Utilities
9.Run Registry Tools by double-tapping it
10.Tap the Reset button
11.Answer Yes when told system will reboot.
When it reboots again, all the registry entries should be cleared to the factory defaults and you should be able to setup a partnership.