About Windows Installer, the .NET Framework, and Visual Studio.
The Microsoft .NET Framework 2.0, Visual Studio 2005, and SQL Server 2005 will soon be shipping. If you'll be installing these products and are planning on upgrading your operating system later be sure to repair the product from the Add/Remove Programs (ARP) control panel.
Some components don't install on certain operating systems. Mostly notably are ASP.NET server-related files on Windows 98 and Me, and Win32 Side-by-Side components on older Windows NT-based platforms. When you upgrade the operating systems you'll most likely want these installed. They aren't installed originally because they are transitive components that are evaluated on each repair (including patching) of the product in addition to the original installation. They would have conditions like VersionNT > 499. Since VersionNT is not defined on Windows 9x the transitive component is not installed.
VersionNT > 499
After you upgrade your OS, find the product in ARP, select it, and click "Change/Remove" for most of our developer products that use an external UI handler. Normally you'd click the "Click here for support information" link and click the "Repair" button if one exists.
This causes the transitive components to be reinstalled and their conditions re-evaluated, installing applicable components and getting rid of those that are no longer applicable. This is just limited to these products but others may have similar issues. If you notice one of your products isn't working correctly after an OS upgrade and it should, try repairing. Do keep in mind that not all installers support repair operations, but Windows Installer does and is much more common these last couple years.
If you're a setup developer, please keep transitive components in mind when installing platform-specific files.