I was wondering whether I can install PCI DAQ card (M series) and an USB DAQ device (M series) in the same computer. If this is possible, do I need to reload the drivers for the PCI card? (I have already installed the USB device)
Yes multiple devices on the same computer is not a problem. As far as the drivers are concerned, I have no idea what you are using currently to communicate with the USB device. If you are using NI DAQmx then you should be fine.