Are you using the following tutorial?
http://forum.ppcgeeks.com/showthread.php?t=24979 If you are and your PC drops USB connection - just wait a few moments and you will see that it comes back. This happened to me 3 times during the ROM updates (parts 5 and 8 from the tutorial) Note that when you at the bootloaded screen ActiveSync won't work - but your display will show 'USB' instead of 'serial' on the bottom. Just be patient, don't rush any steps and don't reboot or disconnect the phone unless it's part of the step. At one point in the ROM setup screen it told me to unplug and plug my phone back in and establish an 'activesync' connection - all you need to do is unplug the cable and plug it back in. Don't worry about it actually establishing an activesync connection - as long as your display shows 'USB'.