Technically it's illegal in a same way as uploading a movie would be, as the games are properties of their authors (and not players) and these authors could sue you
I just got an old 98's Dell Latitude CPt laptop with pirate WinXP Pro SP2 installed (CPU Intel 330 MHz with 192 MB RAM and 5.5 GB HDD). Working surprisingly well. Imagine my face when I discovered what's installed here: http://i.imgur.com/zWa8zbL.png