I've done it with the wondows xp disk and using partition magic. Partition magic is the easiest program to use. Its safe to partition a drive after an OS has been installed, but you can run into problems if you are moving a drive that has an OS installed (not recommended). Also, if you are trying to split a drive that has an OS on it that may cause problems. If this is the case, then just wipe the drive completely, reinstall the OS and partition the drive using the win setup disk.