cell borders change in adjacent cells

I'm using Word 2003, and when working with tables I'm
having some trouble with the cell borders. This table
has a lot of variation from row to row - some rows are
just one cell, and other are split into 5 or 6 cells.
When I try to set specific border properties for some of
the cells, it affects adjacent cells and I can't get the
results I want. For instance, if I am in one of the
cells on the row that contains six cells and set the
bottom border to be a .5 line, the row below that is just
one cell wide will automatically have that type of border
all along the top. The result is that all six cells on
the original row now have a bottom border. Anyone know a
fix for this? Thanks!!