The Microsoft Windows Installer is an application installation and configuration service. Instmsi.exe is the redistributable package for installing or upgrading Windows Installer. Windows Installer is a component of the Windows 2000 operating system that simplifies the application installation process.
Used by Microsoft Windows to manage software packages (like RPM or DEB for Windows). This is the latest version available for Windows XP. Getting this to work would allow LOADS of programs to run on wine.