Why am I unable to synchronize my phone with the computer through the USB cable?

Check the following: Check the Microsoft® ActiveSync® version on your PC. If you have a Windows Mobile® 6 device and a Windows® XP SP2 computer, make sure you have ActiveSync® 4.5 (or later version) installed. Please note that if you are upgrading from an earlier version of ActiveSync®, you may need to recreate the sync partnership with your phone.
If you are using a Windows® Vista® computer, make sure Windows Mobile Device Center is installed on your PC
If ActiveSync® or Windows Mobile Device Center is already installed, check the physical USB cable connection. Make sure the USB cable is properly and securely connected to both your phone and the computer's USB port.
Make sure the Allow USB connections check box is selected in ActiveSync® or Windows Mobile Device Center.
Try unplugging the USB cable, select Start > Settings > Connections > USB to PC and clear the Enable advanced network functionality check box, then reconnect the USB cable. This forces your phone to use a serial connection.
Unplug the USB cable, delete the partnership, reconnect the USB cable, and try recreating the partnership.
While you're installing ActiveSync® 4.5, it will add the necessary exceptions in the Windows Firewall. If you still experience problems with Windows Firewall blocking ActiveSync® connectivity, do the following:
a) On your Windows® XP SP2 PC, click Start > Run.
b) Enter firewall.cpl and click OK.
c) The Windows Firewall's settings dialog box then opens. Clear the Don't Allow Exceptions check box and click OK.
d) If the above process still does not resolve the problem, open the Windows Firewall's settings dialog box again and select Off to turn off the firewall.Sometimes, anti-virus software that is installed on your PC may conflict with ActiveSync®. Disconnect your device, disable the anti-virus software, reconnect your device, and then try synchronizing again.

Source: www.HTC.com