How can I add a complex validation rule to a field in Access 2003 database?

I have a table that I need to add some complex validation in order to keep users from saving a record with invalid data.

Specifically, there is a Taxable field.  If Taxable = No, then there are 4 fields that MUST have something in them before the record can be saved.

So, if Taxable = Yes, record can be saved.

If Taxable = No, record can only be saved if Type <> "" and ExemptNo <>"" and ExpirationDate <>"" and Certificate <> ""

Would I build an expression and put it in the Taxable field?  Or have different expressions in each of the 4 required fields when Taxable = No?

Thanks for any guidance!