USB debugging is turned off
usb debugging must be turned on.
after turning usb debugging on,unchecking "fastboot" in settings/power,plug in the phone and select charge only mode.
now open your cmd window. if
adb devices is returning the serial number,then
adb reboot bootloader. this will take you to fastboot,wich is the screen you want to be on for using fastboot commands in the cmd window.
do NOT push the power button to select bootloader,as this will take you to hboot,and the fastboot comands will not work there.
after your phone boots to fastboot(wich may take several seconds) try
fastboot devices if you get a serial number then,youre good to go.
if not,then it must be some sort of driver issue.
as said above,disable/uninstall htc sync and any other programs capable of communicating with your phone. with your phone plugged in and in fastboot mode,locate the phone in device manager. try installing the drivers manually,as described here:
public:windows_hboot_driver_install [RootWiki] tho keep in mind that guide is for older,mtd devices. you want your phone to be in fastboot,not hboot.
id use these drivers:
http://downloads.unrevoked.com/HTCDriver3.0.0.007.exe
they are a .exe file,so if you havent tried it allready,just try running the .exe and then repeat the steps above to see if you now ger a serial number from
fastboot devices if that doesnt work,you can extract that file and direct windows to that unzipped folder when manually undating them with device manager.
hope that helps