sorry youre having such trouble. is htc sync completely uninstalled? ive never been prompted to install sync manager.
still nothing above and beyond the checklist. one of those things has got to be not correct/changed somehow. if the phone is in fastboot(displaying fastboot usb
at the top),and correct drivers are installed for your OS my gut feeling is something running on the PC. sometimes its a program that wants to communicate with the phone,sometimes its an antivirus,spyware,or similar program.
what windows version/bit system are you using?
have an old laptop or other pc you can try?
if you open device manager,place the phone in fastboot,then plug in... do you get "found device" noises? what pops up in device manager?