This is the link:
http://www.microsoft.com/downloads/details.aspx?FamilyID=0B4F56E4-0CCC-4626-826A-ED2C4C95C871&displaylang=en

You can use this SDK to develop VSS requestors, writers and providers. For example, you can write code that define and use shadow copies on Windows XP and Server 2003. Note that this SDK contains only the headers/libraries for developing VSS applications, plus sample and test code. It does not contain the VSS documentation, which is already available on MSDN here.

After you download the SDK, I would suggest you to start start with the VSHADOW sample application. You can use this application to exercise most of the VSS infrastructure, like creating shadow copies, or assigning drive letters to them. VSHADOW also has the ability to import hardware shadows, or selecting VSS components for backup/restore.

VSHADOW is a console application. The set of VSHADOW command-line options depends on the operating system. Note also that there are two versions of VSHADOW, one compiled for XP and another one for server:

1) For Windows XP you can:
- create, delete, query non-persistent shadow copies
- enumerate writers and their components
- select components for backup/restore, notify writers.
- executing a backup/restore CMD script

2) For Windows Server 2003, all of the above plus:
- Assigning a drive letter, a mount point or a share name to a shadow copy.
- Creating persistent vs. non-persistent shadow copies
- Creating shadow copies without involving writers.
- Hardware shadow copies - creation, import, break, deletion. (only for Advanced Server or Datacenter SKUs)

Finally, VSHADOW allows you to use the VSS infrastructure for a fully automated backup/restore. In this SDK, you can find a set of sample CMD scripts that use VSHADOW to backup & restore of a SQL database.