Symptoms: error message in the installation console:

'RegAsm error: File or assembly name Extensibility, or one of its dependencies, was not found.'

In order for ReSharper installer to work properly, some VS.NET assemblies (like Extensibility) should be in the Global Assembly Cache (GAC). They are normally installed there during VS.NET installation. However, some faulty installers may remove these assemblies from the GAC. So ReSharper installer will be unable to register its own components for Interop and therefore will fail. Workaround is very simple: install necessary assemblies (usually only Extensibility) by running

"[VSInstallDir]\SDK\v1.1\Bin\gacutil.exe" /i "[VSInstallDir]\Common7\IDE\PublicAssemblies\extensibility.dll"

where VSInstallDir is the root VS.NET 2003 installation folder.