Or well planned setups where you've seperated the system/program/temp/different types of user data completely Or on decent non-Windows filing systems

But if you have to defrag, just use the built-in Windows one, that does the job well enough Or if you have enough disk space on another partition (preferably another physical hard drive), it's much faster to copy all the data to the spare partition, reformat the partition you want to defrag, then move everything back, especially for very large partitions