Diskeeper Ver 9 supported drives 100G or Lower. Diskeeper ver 10 removed that barrier and supports drives 60G or larger. Ver 10 runs into the Terrabyte range. There is no indication what size the built in defrag in XP will support. This defrag was built by Diskeeper and has been in use for several years. I do not know if MS patches have upgraded this version or whether it runs in it's native format. If it is running in the native format, my guess is that it will fall into the 100G or lower capacity mentioned above.