i've had this problem for a long time
** Z_Malloc: failed on allocation of 4194328 bytes from the main zone
Please report the the problem you encountered on the Quake Live forums.
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I did no modifications to the game, no strange console commands, or anything.
I would uninstall quake live delete the quake live folder, re-download quake live let it reinstall everything, sounds like a file system issue or one of the files got corrupt somehow. also run a chkdsk make sure your not having any hard drive related errors.