Install current Device Drivers from the USB to Serial Adapter manufacturer's web site. The Elecraft KUSB uses drivers from http://www.prolific.com.tw/eng/downloads.asp?ID=31. The chip set in the KUSB is a Prolific Technology, Inc. PL-2303.
After installing new device drivers, take a moment to verify that the new device driver was actually installed. See USB to Serial Adapter Troubleshooting.
Users of Microsoft Vista can obtain acceptable Prolific device drivers from Microsoft Update when the KUSB is first plugged in.