As was previously mentioned, make sure that you are in the bootloader when running fastboot.
Now, when I updated to Android 4.3, my very old SDK kit had several programs that did not work anymore (Some of the commands didn't work, USB driver didn't work, etc.). So I installed the latest SDK, but later noticed that there was an update manager built right into the kit. It is located in the folder that has a name similar to the following: adt-bundle-windows-x86_64-20130717 (The name may look a little different), and the name of the program is "SDK Manager.exe". I ran it and several updates appeared, so I clicked on the Install button. The Windows driver is hidden under the "Extras" section and listed as "Google USB Driver". When you highlight that item it shows where it is located in Windows on the hard drive. The current version of this driver is "revision 8". After removing the old driver from the Device Manager in Windows, as I recall, I had to use the "search for the driver" mode, as pointing directly to the driver did not work for some reason. After installing it into Windows, it was listed as "Android Bootloader Interface", Version 184.108.40.206, dated 07-09-2013. These various updates solved all my 4.3 problems. What is really nice about the SDK is that you don't have to rely on drivers and programs posted on a forum somewhere that you have no idea whether they are up to date or not, and for that same reason and really understanding how things work and how to troubleshoot from the command line, is why I would never use a toolkit.