It is very easy to partition a HD:
1. 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.
2. Right click “Computer” and choose "Management" to get into Disk Management.Now you can see a list of your existing disks.
3. Right-click the disk, and select Shrink Volume.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.
4. 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.