About.com offers a tutorial on partitioning a hard drive in Windows 7, and OSXDaily.com offers a disk partitioning tutorial for Apple's OS X. Back up your hard disk before following either tutorial, in case something goes wrong. Each tutorial takes a few minutes to complete.
In Windows 7, open Disk Management, and select the desired hard disk. Right-click on the unallocated space, and select New Simple Volume. Specify the partition's size, and assign a letter name to the disk. Formatting at this stage is optional. However, once the partition is created, it must be formatted.
In OS X, open Disk Utility. Select the hard disk, and click Partition and the plus sign. Name the new partition, and select a file system before clicking Apply.