It shouldn't be nessacery with any reasonablly modern tools including those included in d-i. Indeed windows XP defrag does not consolidate free space anyway so using it before install is really just a waste of time.While preparing this, I noticed that there is no hint in the manual, that in such cases man should use the defragprogram to put all data on the ntfs partition together to one block.(defrag program is only mentioned in chapter about non-debian partitioning, but using defrag is also important, when d-i ifself does the partitioning job, isn't it?)