USB to PC can be found in Settings/Connections. It is only relevant to AS 4 onwards and WM5 onwards, so older devices aren't affected.
Turning off the 'Advanced Connectivity' reverts the connection to simple USB rather than RNDIS; some versions of Windows connect better this way, and a stalled connection will often work.
If that doesn't work, you may need to do one of 2 things: a hard reset of teh device if the connectivity has become corrupted, or a full removal/reinstall of A/S on the PC. I'd try that 1st as its less detructive, but 1st I'd go into both the PC and PPC ActiveSync settings, and Delete every current partnership, then reconnect the PPC and see if it tries to create a new one.
If that won't work, go to the PC and uninstall ActiveSync. You must also manually remove the Program Files/Microsoft ActiveSync folder otehrwise remnants of teh old installation will remin. Now obtain a new version of A/S from Microsoft and install it, then reconnect. Be aware that connecting a PPC before installing ActvieSync will always fail as A/S contains the device drivers.