A lot of programs are to the taste of the user. Obviously you know Windows lol.
You should get either Firefox or Google Chrome for web browsing, don't use Internet Explorer.
Adobe Flash player.
VLC Media Player is a fast, small, and powerful media player that can play pretty much any sort of video.
MusicBee is a really good program for listening and managing your music files, unless you have an Apple device and prefer to use Itunes.
Malwarebytes is an excellent antivirus program, with a small problem that it doesn't have real time protection in the free version. In other words, it doesn't do anything to stop viruses from getting on the computer, its just amazing at removing them. However the Pro Edition is only about $25, has a life time license that can be transferred from computer to computer and includes Real-time Protection. If you have a few extra bucks I would highly recommend it.
If not, Panda Cloud is also good.
If you play games, you should probably go ahead and install Steam, a gaming service and game store that is essential to PC gaming now.
Fox it PDF reader, a free PDF reader that is nice and easy to use.
LibreOffice a free word processor similar to Microsoft Office.
That is about all the big things I can think of.