You can bump your files onto a disk, then delete them off your hard drive and then restore them after defrag, this will enable certain files to be less fragmented.
Delete your system restore points(system restore can take quite a bit of space, and will not be touched by the defrag option), before a defrag, then reenable the function and create a new restore piont afterward.
You can perform the defag option with a pitance, 15% is the message you get as the recomemended value, you should always do this a least twice.