OpenERP comes with a generic payroll engine. Configuring it for a particular country is possible but requires not only extensive legislation knowledge but also technical knowledge of OpenERP. A ready-to-use module that would configure the engine for the US doesn't exist at this time (in fact it barely exists for a handful of countries at this time). Due to the changing nature of payroll regulations, if you are a small business you are much better off outsourcing your payroll to ADP or a similar HR service provider than taking care of it yourself. The burden is too high from a legislation standpoint and the risk is not worth it.