Just guessing this is what you need.
1) right-click desktop... change the DPI scaling if you want but for 1600x900 I'd stay with default
2) open CLEARTYPE if you have software with text issues
3) SCROLL + ALT to change browser page scaling
Other:
If you haven't done so the monitor or GPU should be set to scale by aspect for games with a different resolution. For example, on the monitor itself if you set scale by aspect, then if you had a 1024x768 program it should fit the HEIGHT of your monitor (1600x900) and have black bars on left and right, but would NOT be stretched.