Mac OS X spotlight is a cool service but it may extremely slow down your system performance if you have an older system or are dealing with lots of files. It keeps the harddisk extremely busy leaving no time for normal system operation.

To permanentely turn Spotlight indexing off the following commands need to be executed in a terminal session:

sudo mdutil -a -i off
sudo chmod 600 /System/Library/CoreServices/Search.bundle/Contents/MacOS/Search

The menu bar will be restarted with

killall SystemUIServer 

It might be necessary to reapply this procedure after a system patch has been installed.