I use Orca from the Windows Installer SDK to view the contents of Windows Installer setup packages (MSI files) and Windows Installer patch packages (MSP files) all the time. When I view MSPs, I typically right-click on the applicable MSI, choose Edit with Orca, and then drag-and-drop the MSP file(s) into the Orca window to view the changes that the patch will make to the original MSI.
However, I recently ran into an issue while using Orca on Windows Vista - dragging and dropping MSPs onto my MSI in Orca does not load the MSP. Instead, I am only able to open the MSP by going to Orca's Transform menu, choosing View Patch... and browsing to the MSP file in the Open File dialog.
As I previously mentioned, one of the features that I used to use all of the time in previous versions of Windows no longer works in Windows Vista - dragging and dropping a file or folder into a cmd prompt in order to copy the path to that file/folder into the cmd prompt. It appears that there is some kind of security-related limitation regarding dragging and dropping objects in Windows Vista that affects my cmd prompt scenario and also dragging MSPs onto an MSI in Orca.
In case you run into problems viewing MSPs in Orca on Windows Vista, just make sure that you use the Transform | View Patch... menu option instead of relying on dragging and dropping like I used to.
Also, Heath Stewart has a blog post at http://blogs.msdn.com/heaths/archive/2006/11/07/viewing-patches-in-orca.aspx where he shows some screenshots of what MSPs look like when you apply them to an MSI within Orca.