First, back up your data. The partitioning process shouldn't delete any data, but you can't be too careful when working with your hard drive.
Open the Disk Management tool by slecting Control Panel, Administrative Tools, Computer Management. An entry for Disk Management should show up underneath the Storage heading on the left-hand side.
Click Disk Management, and you should see a list of your existing drives and partitions (if applicable). To create a new partition, choose a disk that has free storage space, right-click it, and select Shrink Volume. Bear in mind that you can safely shrink only basic disks; if you try to shrink dynamic disks (such as RAID arrays) that are formatted with the NTFS file system, you'll risk losing your data.
Type in the amount of space you want to shrink the disk by. The number you choose depends on what you want to use the other partition for. If you plan to dual-boot another version of Windows, for example, you'll want a lot of space (Windows 7 requires at least 16GB of available space, for example); but if you're setting aside room for an emergency recovery partition, you won't need nearly as much space.
If you plan to install an OS in this partition (Windows or otherwise), you'll probably have to stop here--your operating system installer will likely insist on making the partition itself during the installation process.
Once you've cleared the necessary space, it should show up in the Disk Management window as Unallocated Space. Right-click this entry, select New Simple Volume, and let the resulting wizard guide you through the remaiing steps of the process. Generally, you'll want to format the new partition as NTFS; and unless you're using this partition for archiving purposes, you won't need to enable file and folder compression for the whole drive. (If you decide to enable compression later, you can do it easily enough: In the Disk Management tool, right-click the partition and select Properties, Compress this drive to save space.)
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