I am trying to make three drop boxes, each containing a percentage eith an
incremation of 5% (5%,10%,15% etc.). I would like the second to be
affected
by the first so the total selectable total is never over 100% (ie if box 1
says 10%, I can select 5% to 90%in box two), and the third to be affected
similarly by the second and first. As the first box is filled first it
need
not be prohibited by the others.

To do this I am using an IF formula within the validation, but as I can
only
use 7 IF Statements I can only get to a condition in which the first box
displays 35%. Is there a way to further stack the IF statements (either
within excel or using macros and visual basic), or is there a simpler way
to
solve this problem?

Let's say your 3 cells are A1, B1 and C1, then try these validation formulas
in Data/Validation/Custom...

A1: =MOD(A1,5)=0

B1: =AND(A1+B1<=100,MOD(B1,5)=0)

C1: =AND(A1+B1+C1<=100,MOD(C1,5)=0)

Rick