When you say "free PC software" it immediately associates me of Linux OS, not really just a single application. You get a free OS, and with it all the free to use applications you want. But we're talking Windows OS, which I install and maintain on PCs I assemble alongside Linux OS, then some of the essential apps I install and think everybody should use them are: CCleaner, Firefox ESR + Thunderbird nad Lightning if needed, XNView for viewing pictures, Media Player Classic HC [stands for "hard core" ] for viewing videos, GIMP for picture/photo editing, Wavosaur for audio recording and editing, VirtualDub for video editing, and Abiword [2.8.6 last] for making and editing RTF and DOC documents.
Sometimes I add VLC and LibreOffice to the mix. "Stickies" free app if you need - stickies. And if that's not enough I add TreeLine PIM to the mix for notes. You don't really need more than that for your everyday usage.