If you are using Windows Vista 32bit System then try the following, I had the same issue as you and this worked for me :
The USB debugging check box in Home > Menu > Settings > Applications > Development needs to be selected for the phone to be able to synchronize with the PC using HTC Sync. On your computer, run HTC Sync. Connect the phone to the computer using the provided USB cable. The first time you synchronize the phone, the Synchronization Settings Wizard starts automatically to help you set up HTC Sync. Select the PC application you want to synchronize with and then select the categories you want to synchronize. Select when to synchronize data and then check the settings that you have chosen. OR You can always go to this page listed below here for support on different ?'s :
http://www.htc.com/us/support/droid-eris-verizon/help/synchronization
