Had the same problem. I discovered that the drivers were stored in a different folder than they should be. The install wizard stored them under C:\Programfiler\HTC\HTCDriver\Driver Files\XP_x86, while the sync proram searched C:\Program Files\...
Solved it by making similar folders (empty) under C:\Program Files, and then copying the XP_x86 folder with all its contents to the C:\....\Driver Files
If you don't use english language on your computer, you might want to check if you have a similar problem.
Also, check that you havent accidentally turned off the PC Synchronisation app on your phone. Reboot (turn off and on) your phone to be sure.
Good luck!