The solution is rather quite simple.
The large amount of dust in your PS2 is causing it to overheat. When the PS2 overheats, the disk you have in it warps, and touches the PS2 lens, scratching the disk, making a circle mark on the disk.
Clean the dust, then clean the lens with a PS2 lens cleaner CD.
As for the circle marks, take a wash rag, put some alcohol on the end of it, and rub from the inside out on the disk until the circle \'scratch\' which really is just a burn mark fades away.