If you’re attempting to install the Visual Studio 2005 Service Pack 1 Update for Windows Vista without having installed Visual Studio 2005 Service Pack 1 first on all applicable products, you will receive Windows error 1642, which reads,

The installer cannot install the upgrade patch because the program being upgraded may be missing or the upgrade patch updates a different version of the program. Verify that the program to be upgraded exists on your computer and that you have the correct upgrade patch.

Some news outlets and blogs have reported that this would be or is a special edition of VS 2005 SP1 but it is actually a hotfix that installs on top of, and thus requires, Visual Studio 2005 SP1. The system requirements in KB929470 and on the download page also state that SP1 is required.

Also note that each localized hotfix targets more products than SP1. VS 2005 SP1 was split up into multiple patches to prevent the patch package from being even larger than it is. This is because of how files are shared across products and stored within a patch package. VS 2005 SP1 targets separately,

• Localized versions of Standard, Professional, Team Suite (and the individual component SKUs, including Architect, Developer, and Tester), and co-installed products like the Test Agent and Controller.
• Localized versions of Team Foundation Server
• All languages for each particular Express SKU (i.e., all Visual C# 2005 Express localized editions, all Visual Basic 2005 Express localized editions, etc.)

So, if you have both a VS Express SKU and a VS SKU installed, you’ll need to download SP1 patch packages for each and install them first. Each patch may appear to install multiple times because many of our SKUs are comprised of multiple products that are each patched. Then install the Visual Studio 2005 Service Pack 1 Update for Windows Vista. If you later install another SKU, you will need to re-install both SP1 and the Vista update for that SKU.