Hi, Karen.
Use Disk Management. One way to find it is to type at the Run prompt:
diskmgmt.msc
BUT...DM cannot shrink or otherwise change the size of an existing
partition. Microsoft does not sell anything that will do this. You'll need
a third-party program such as Partition Magic to do that job. The only way
to do it with DM is to delete one or more existing partitions or logical
drives and create one or more new ones in that space. And DM cannot do
anything with the System Partition or Boot Volume.
You said "an existing HD". But the answer is different if that HD has
already been partitioned, especially if ALL the space on the HD is already
included in an existing partition. But if you have unpartitioned space on
that HD, then Disk Management is the tool you should use.
RC