I'm trying to make rounded walls for my map like a like a very large circle wall. Can anyone help??

Unfortunatly, your idea doesn't fit too well with the general artistry style of XP tilesets (that is sharp corners, being only one square in size), so you would be hard pressed to find a tileset like that.

I recomend the resource request section of the forums, and if that fails, attempt to create this effect yourself.

Is there any sort of example you can show us as to the scale of roundedness you'd like to achieve?
Do you have any experience with creating/editing your own tilesets?