If you buy an ePub or mobi file, they come to you already unzipped. Move it right on over to your device.
If you want to read it on your computer, first install (for ePub) Calibre (a free download), and assign Calibre to open any ePub file; or, for mobi, get Kindle Previewer (a free download from Amazon) for your computer, and assign Kindle Previewer to open the mobi file type. After you have one of those two softwares on your computer, and once you have ‘assigned’ a file type to be read by THAT program, it will always ‘read’ immediately after you click on the file.
TO ASSIGN a program to read a particular file type, go to the Control Panel on your Start menu (icon at extreme left of your bottom toolbar on Win 7) and click on the option “DEFAULT PROGRAMS”. That lets you set which program automatically comes up to handle a particular file. Once you have ‘assigned’ a program to open a particular file type, click on the file, and that program will automatically come handle it.