This is a very common problem when using booleans for modeling, or when exporting from a modeling software to Poser. Ockham's sugestion is one of the easiest ways to control the ceation of narrow triangles. Depending on the software you are using, there may be some other options to fiX the model without recreating your arches/windows (but it may be just as much work). 

One option that may work if your modeling software has the ability to "cut" the mesh, you can create a square/rectangular frame enclosing the problem areas, then force (weld) the triangle verticies at the new corners. You will still have a lot of triangles, but they will be shorter and usually less of a problem in Poser.

Another option would be to manually re-cut the model with lines extending perpendicular to the edges from the arch/window to the edge, then either manually merge the triangles into quads, or if  your software supports it, you can do an auto convert from triangle to quads.

Unfortunately, I don't know of an easy way to "fix" the model. If you could let us know what software you used to create the model, maybe one of the gurus could give you a simple fix. There could be an option within the software to force the creation of quads, which would be the easiest fix.

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