Command & Conquer Red Alert 2 is a classic real-time strategy game, first released for Windows back in 2000.
The two bundled campaigns - Red Alert and Yuri's Revenge - follow on from the first game, and pitch the Allies and the Soviet Union against each other in a range of missions from small and stealthy to fast-paced, full-scale multi-unit battles.
There are also various smaller "scenarios", quick skirmishes which you can use to learn the game or if you've only a few minutes to spare.
This isn't a trial or restricted version, it's the complete product. The only "catch" is you'll need an Electronic Arts account and a copy of Origin to download it, but that doesn't involve much more than providing your email address, and everything else is handled more or less automatically. (Although it might still take a while, as the download is a chunky 1.6GB.)
The game should work on any modern PC - and most ancient ones, too - but keep in mind that it expects to be running full-screen and exclusively. If you Alt+Tab away to some other application, the game screen may not be displayed correctly when you switch back.