Check this troubleshooting guide from MS if you are using USB for syncing
http://www.microsoft.com/windowsmobi...c/default.mspx
Security applications (e.g. desktop firewalls, antivirus, IDSs, etc.) may cause Activesync to stop working properly.
I had problems with ISS Proventia Desktop. I added the Activesync files (Wcesmgr.exe, Wcescomm.exe, Rapimgr.exe) as trusted applications (allowed to communicate through the firewall) and I created rules for the ports mentioned in the guide (incoming TCP 990, 999, 5678, 5721, 26675 and outgoing UDP 5679). The source IP address range is 169.254.2.0/255.255.255.0 (in order to not allow everybody. I found it in the automatic rules that Activesync creates in Windows XP Firewall). After these changes, everything is working fine.
Hope it helps.
Apakoh