There is a yellow box in the middle of my computer screen, it won't go away and I don't know what it is for...

There is a red circular icon at the left of he screen, then a green bar with a blinking cursor, below that it says 100%... what the heck?

Try turning off the monitor, and then your computer. Unplug everything, restart, reboot- if you are still getting the same problem then you should contact the vendor... it sounds like the monitor settings have been skewn; if so a simple fix. Just use the adjustment buttons.