validation error

I have a Custom Object configured as "Setup Forms" with a dynamic layout driven by Task Type. 

We have 2 "Form Types"

- Customer Setup

- Supplier Setup

They can do multiple Task Types under each form type and it is a cascading picklist with Form Type driving Task Type.

Since it is a custom object, we do not have the address fields. We created a Country picklist and the other standard address fields.

Now I need some validation for a specific scenario

(Form = Supplier Setup; Country = USA and TAX ID should not be NULL ) OR (Form = Supplier Setup; Country <> USA and Currency should not be NULL)