You could check that the phone is in Mass Storage mode (I think the S would still be that rather than MTP)
If that's not it, then it's most likely a dodgy USB cable - try a different one.
Otherwise, it will be drivers:
- disconnect the phone from the computer
- uninstall ALL phone related sofware and drivers from the computer
- restart the computer
- restart the phone
Re-install HTC Sync - do NOT connect the phone until prompted
Do NOT disconnect the phone until ALL drivers have successfully installed.
