MINIX 3 OS is an operating system. If you wish to retain your existing operating system and data (recommended) and create a dual-boot machine, you will need to partition your hard drive. You may use one of the following:
Partition Magic (http://www.powerquest.com/partitionmagic)
or
The Partition Resizer (http://www.zeleps.com)
Follow the instructions at http://www.minix3.org/partitions.html