Microsoft: A software company that introduced us to an unnecessary and risky extenstion to their browser AFTER JAVE was made a standard.
ActiveX: The browser "plug-in" that lets it download and run native code provided by any old web site.
Brilliance!
All versions of Windows will let you replace a system .DLL by simply putting something with the same name in the search path for your application. Handy, if you want to get something nasty invoked when a certain kind of library is loaded. Such as by copying something into a file from "safe" script code and letting it get inadvertently invoked the next time your email client (or some other software) is run.
Sure, everyone is concentrating on Windows for viruses. There is absolutely no difference to Windows between a data file and a native executable image. Any process with any level of permission may "create" a file under Windows, and let it be invoked by an inadvertent reference as an executable image later, or with a quick "tweak" in the registry. Paradise for a virus writer.
Then it can start modifying other things "for you". After all, everything is given the user's implied consent under Windows.
It would be "way too much work" to provide a special mode for Windows to change what is installed, and register what is executable, and protect these critical things from any modifications outside of this mode. People who ship software might have to (gasp!) have a directory set aside for the executables while installing, or flag some files to be "executable ones" and test it.