Try to repair the disk with fsck or Disk Utility. Your NetInfo database is probably corrupted.
If this this not enough, you can try to delete and recreate the netinfo database by doing this :
Boot OS X in single user mode (hold down cmd-S during startup), and then enter the following commands :
mount -uw /
rm -rf /var/db/netinfo
rm -f /var/db/.AppleSetupDone
sync
Then restart, you will be prompted to recreate your user (this will not erase any info on the disk)
edit: simplified the response
