1. A computer-aided design system for converting a stored geometrical representation of a structure, the material properties thereof and the loads imposed thereon into a visualization of a mechanical quantity of the structure, said system comprising: means to generate a mesh from said stored geometrical representation of the structure, the material properties thereof and the loads imposed thereon, said mesh describing the structure; means to generate, by application of the finite element method to the mesh, the elements of a stiffness matrix, a loading vector, and a vector including an associated degree of freedom index for each row and column of the stiffness matrix; means to generate from the stiffness matrix and the loading vector a matrix A and a right hand-side vector f, wherein any restrained degrees of freedom have been eliminated from the stiffness matrix, and where A and f are related through the relation Ax=f, the vector x representing the mechanical quantity at a plurality of points of the mesh; a linear solver of the preconditioned conjugate gradient type for generating the elements of the vector x from A and f using a preconditioning matrix K, wherein the linear solver comprises means to generate a principal submatrix from the stiffness matrix by setting to zero those elements therein in positions such that the degree of freedom index associated with their row is not equal to the degree of freedom index associated with their column and means to generate the preconditioning matrix K from the principal submatrix; and means for visualizing the mechanical quantity of the structure on the computer-aided design system as a function of said vector x.