Really, it exposes the flaws we mentioned earlier in instructions. Difficult to think of everything.

(86) refers to 586 or 486, etc... Really it just means "hey this is OLD stuff!!" Windows 7 likes to be 64 bit, but has a 32 bit compatibility mode for the old stuff.

Ok, so you found the Arduino folder. But we are not dragging and dropping the files.

You want to select the 3 folders in the library folder, right click, and pick COPY. Then go to the Arduino folder, open the libraries folder, and then right click on some white space and choose PASTE.

You are adding those three folders from the Marlin download to the libraries folder in the Arduino.

Does the Arduino folder also have another folder named projects? That might be something also different on Windows 7.