this is one reason i always have folks enter
fastboot devices prior to running any other commands,as it will tell you if your device is present or not
. sounds like you have some sort of driver or interfearance issue from another program.
for starters,i recomend these drivers from revolutionary:
http://downloads.unrevoked.com/HTCDriver3.0.0.007.exe
simply download and run the .exe file
them make 100% sure that you have
every program capable of comunications with the phone temprarily disabled/uninstalled. this incudes htc sync,pda net,easy tether,even itunes has been known to cause prollems.
ive learned the hard way
eek
that not all usb cables are created equal,so try different cables,different ports,and make sure youre not using usb 3.0 on the pc or for accessories.
if youre still having trouble after that,you can plug in the phone in fastboot,open device manager and find the phone. right click on it,then manually update the drivers,pointing the wizrd to the
extracted driver file that you downloaded above. if the wizard indicates your drivers are up to date,then you may try with a different PCor laptop. unfortunately,some folks do not seem to be able to track down the program/s that are causing the prollem with connectivity,and find the only way they can get adb/fastboot to work is to switch to an entirely different machine,preferably one that has not had ANY phone related things on it