What is safest remote connection from home to a small business?

Hi
My friend has a small business with a Dell desktop running XP. The Dell is currently connected to a Belkin router/wireless which is connected to the cable modem.

She would like to connect from home to the office using her iPhone/iPad. The Dell must stay at XP until the business software releases a version for other than XP. There are also a few laptops that access the business software but do not need Internet access.

What is the safest way to configure her office so she can remotely connect? I had planned on removing the Belkin as wireless does not work in the old building.

thanks