Windows should find a driver for 5.1. Go to the control panel and check the device manager. Any device driver that didn't load properly will have a mark next to it. Let windows search for an update while online if your speakers aren't working, but check all connections first.
Speakers do not require a software driver. Actually, there is no such thing as a windows driver for any speakers. All you require is drivers for your sound card. Then get into the windows sound options and set it to 5.1 as default.