Symantec has a Klez removal tool that is supposed to remove all versions of Klez. Get it
Here
More info about Klez and what it does
Here
ah, it does disable Norton, and AVG.
I would say the best bet is to download the Klez removal tool on a PC that's not infected, create a boot disk, copy the FixKlez program to that disk, then boot up the infected PC with the boot disk and run Klez.
Be sure to check out the command options for the Klez removal tool before using.