The capability of the contemporary computer as a recording device becomes increasingly popular as people apply computer recording to new applications. Recording live performance from a PA system is simple with a laptop and many audio mixing boards incorporate USB interfaces within the boards themselves, making laptop recording a one-cable connection. Digital-to-audio conversion offers clean sound that isn't prone to noise induced by the laptop itself.
Connect your USB audio interface between the laptop and the PA mixer. Note that audio interfaces are available in a number of formats from separate boxes to dongles, so a USB cable may be needed between the device and laptop, and audio cables needed between the device and the PA. Contemporary mixers often have a USB connection for input and output, in which case only a USB cable from mixer to computer is needed.
Select your USB device as your recording device. In Windows, right-click the volume icon in the lower right and select "Recording Devices." If the name of the device does not appear, look for an item called "USB Audio Codec," select it and click "OK."
Open your recording software and adjust the input level of the signal from the PA to set record levels on the laptop. Routing of output to the laptop will vary between mixers and connection methods. See the PA mixer documentation for recommended connections.