I can only think of one situation that matches your description. We have found that some of the USB devices distributed with Service Center 8 and older can experience issues on systems running ESET antivirus software.
While we have found a permanent solution, it requires an upgrade to a Service Center 9 USB device, the only other confirmed solution is to remove the ESET software from the computer while testing. You should however still be able to run Service Center DOS (or Bootable Diagnostics if you are eliigble for a free software upgrade to Service Center 9).
Please let me know if you have any other questions or issues, or if you find that the systems in question are not using ESET software.
Thank you,
Kris