I have had exactly the same problem with my HTC Touch Dual. The sync with Windows Mobile Device Center is hit and miss. On some days it will sync without any trouble at all and on other days the phone refuses to sync informing that the PC must have at least ActiveSync 4 installed. I contacted HTC who talked me through many settings and they are baffled.
A reboot of the PC doesn't help but a complete shut down and restart may get the phone to sync.