To locate your etkey file navigate to one of the following:
*Based on Ubuntu setup.Windows 32bit: C:\Program Files\Wolfenstein - Enemy Territory\etmain\
Windows 64bit: C:\Program Files (x86)\Wolfenstein - Enemy Territory\etmain\
Mac OS X: /Users/username/Library/Application Support/Wolfenstein ET/etmain/ (Thanks Mattle!)
Linux (*): /home/username/.etwolf/etmain/
In the etmain folder, you should find a file called etkey.
Best way of backing up a copy of your etkey: Copy your etkey to a memory card or flash drive that way you always have a copy of it.
To Restore: Go to your memory or flashdrive and copy your key and place it back into your et folder where you copied it from.
Following this method you should never have to change your GUID because of a silly computer problem.