I thought that lowest resolution is 800x600,
But I discovered that if I go to "display settings", then "advanced display settings", then "display adapter properties",
There is an option thereĀ which says "List All Modes" and you can select 640x480 screen resolution there.
Whats even better is that after you select that resolution, if you go to "Display settings" again, new options appear where you can select even lower resolution.
This was important for me because I play some games which require like 16 gb RAM like Room Girl, but I only have 8gb.
Lower resolution makes them somewhat playable, otherwise the game lags way too much.