On XP Machines I use O&ODeFrag v8.6
On Vista Machines however, I would not recommend it, as its just too bloody slow - especially v10, it takes yonks just to gather the Disks info and if like me you have lots of disks and lots of partitions, it can be over half an hour just for it to start the process off!!!
v8.6 is the best compromise ( For me ) to work with Vista and XP in both 32 and 64 Bit flavours.