I recently installed Windows XP 64 on one of my machines the other day.  On that ... I have to say my reviews so far are mixed.  I love the fact it recognizes my 4GB of RAM and so far the performance is tight.  I have had a few driver issues ... but have a good handle on it.  Anyway, since installing XP 64 I have been looking for 64 bit applications.  One that I found is UltraDefrag v 1.3.4 which will run with 64 and 32 bit Operating Systems.  This is an Open Source no frills application that gets the job done and done quickly.  As with the native Windows defrag utility UltraDefrag has an analyzing feature.   There is also a legend in the about section of UltraDefrag which explains the various colors on the cluster map.

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I like this defrag utility because it is only a 300KB download, and also includes a portable edition for your flash drive that does not require an install.  This is very handy for PC repair.  There are a few options to the settings to change around as well.

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Download UltraDefrag