This is only true if your action affects either your boot partition
(fairly obviously!) or a partition on which you have installed one or
more _programs_. It is perfectly safe to use XP's Disk Manager to
manipulate _data_ partitions, as long as you subsequently tell any
programs that use them where their data has been moved to.
With care, you can even mess with partitions with installed programs
on them, provided that they are not running at the time (!) and the
installation device and directory ends up (at least apparently) where
it started in the system's tree structure. XP's ability to name
partitions independently of where they physically are can help with
this.