The defrag program provided as part of Windows XP is a greatly simplified version of Diskeeper. If you install Diskeeper 7 it will replace the Windows version and you can set it to run in the background, so you never have to do a manual defrag again. I have been using Diskeeper 7 for a while now and would thoroughly recommend it.
Defragging is a vastly overrated exercise IMHO. Modern systems are fast enough to take a few fragmented files in their strides. Remember, that after a program is installed and defragmented, it can never become fragmented again - and your data is by and large in too small chunks to ever get fragmented in the first place.
For an average user, I never suggest more than once every 3-6 months.
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