I sometimes don't agree with all the advice on this board, but Molly's advice is downright wrong. Payroll is much more than calculating net pay.
The reason you need a system is to track all the additional forms at the federal, state and local levels that need to be filed. Some payments can be online each per-period, monthly, quarterly and for FUTA - annually. The pay schedule is not always the same as the form due dates. And the complexities don't end there. In NY, we don't have to remit withheld taxes until 5 days after the paydate that cuased the total taxes withheld and the SUTA taxes to reach $700 - which for small business could easily be several pay-periods. Try to be late with a payment made up of taxes you withheld and you'll see that the payment and filing reminders are worth the cost of any solution.
You do need a payroll "system". But that does not need to be going out to buy new software. If you are using QuickBooks - you can add on the payroll service starting at $25 per month. That service can e-file and e-pay all the additional tax returns. Most CPAs, like myself also process payroll. I tend to think that this is money more that well spent.