What solitaire card game has been included in the windows operating system since 1990?

The solitaire card game included in Windows operating systems since 1990 is "Klondike".

While the name "Solitaire" is often used generically to refer to any single-player card game, the specific version included with Windows is a variant of Klondike. It features the classic gameplay of drawing cards one at a time from the stock, building up foundations in suit order, and using the tableau to move cards.