let's reset all your tax calculations. This will wipe out all your tax rates, zones, zones definitions, etc.
Then you'll need to re-build them.
paste the following into your Admin->Tools->Install SQL Patches:
Code:
truncate table geo_zones;
truncate table zones_to_geo_zones;
truncate table tax_rates;
update configuration set configuration_value='0' where configuration_key='TAX_DECIMAL_PLACES';
update configuration set configuration_value='false' where configuration_key='DISPLAY_PRICE_WITH_TAX';
update configuration set configuration_value='false' where configuration_key='DISPLAY_PRICE_WITH_TAX_ADMIN';
update configuration set configuration_value='Shipping' where configuration_key='STORE_PRODUCT_TAX_BASIS';
update configuration set configuration_value='Shipping' where configuration_key='STORE_SHIPPING_TAX_BASIS';
update configuration set configuration_value='0' where configuration_key='STORE_TAX_DISPLAY_STATUS';
This doesn't touch your tax "classes", so your products which are already linked to that class will not be affected.