From the documentation Excel 2000 (one I currently have
loaded) can support up to 7 nested IF statements.

To use Logical AND, OR, NOT - they are themselves
functions so you simply do AND( condition1, condition2...)

Finally an example of how you would write that same
statement is (broken up on the false condition for easier
reading):

=if(z67 < 7000, z67* 0.10,
if( and( z67<=28400,z67 =7000),((z67-7000)*0.15)+700,
if( and( z67<=68000,z67 =28400),((z67-28400)*0.25)+ 3900,
and so on...