Partition Assistant Home is software which can resize Windows partitions.
In many cases, new or second operating systems are installed onto a secondary partition in which a boot manager can decide which partition to boot off of. All too often, there is either too little or absolutely no space left on a hard drive in order to accomplish dual-booting.
In other cases, multiple partitions can also serve as a place to store backups. With software designed to resize partitions, users are given the option to resize their current Windows partition in order to allow space for a secondary operating system (such as Linux). The partition managers are those which are fully capable of resizing a currently in-use partition.