[peepeesea]

first thing you want to do is determine if its your computer or the phone
see if you can get it connected on another machine with a different cable
if not then its probably the phone, but my guess its just a driver issue on your machine or even a bad cable or usb port

as "youmaylose" said active sync will say no connected but you can still run the ruu and flash the device.

question, on the bootloader screen on your phone, does it say "usb" in the white part at the bottom?
my mogul some times wouldn't say that so i just had to unplug the usb cable from the phone then plug it back in and then "usb" would show up and everything would start working