Another suggestion, to go along with the outlining:

Either use separate columns for each level of the outline to more clearly
show sub-components (i.e. Column A is for major assemblies, Column B is for
the major sub-components of that assy, Column C is for the parts of that
sub-component, etc), or use the indention feature of Excel to indent cells
that have the sub-components in them. That will also give your outline the
"look and feel" of Windows Explorer by making the difference between the
assemblies and the sub-components more visual.

And keep in mind that you can group columns as well as rows...

Eric