I don't offer payroll as a service right now, but have been considering it for the future, so I havent looked into to many options yet but....
With my regular JOB I use QB payroll which costs about $400 a year for up to 10 employees, this allows you to process and print payroll checks and process payroll taxes. It also keeps you completely up to date with payroll tax tables. I'm not sure yet how well that would work for us Virtual Offices.
The other software program I found that I've considered using in my Virtual Biz is
Paycheck Manager They have a free option called the Paycheck Calculator which you can use to figure taxes and print paychecks, none of your information is kept on their site. The other option is The Paycheck Manager which figures, and prints checks, reports, ect.
I wish I could tell you more, but I haven't researched it completely yet. I'm sure others will have some idea's for you too