You will need an Ethernet hub, not a switch (available cheap on eBay) and a second computer. Plug the Win8.1 computer into the hub and plug your Internet connection also into the hub. The Win8.1 computer now has connectivity through the hub.
Download Wireshark to the second computer and install it. Plug the second computer also into the hub, start Wireshark, and put it into promiscuous mode. Set up a filter to look only at the IP address of the Win8.1 computer.
You will now be able to monitor all the incoming and outgoing traffic on that computer. Do know that there's normal "chatter" even when a computer is supposedly idle, so you're going to have to try to separate that chatter from any malicious traffic.
If the suspect computer is part of a botnet, you may see a very large volume of traffic. That's bad. If it's running a keylogger or such, perhaps not. Log on to an unimportant site and See What Happens. (tm) (Change your password on "unimportant site" immediately after, and from a different computer.)
If you find evidence of malware, there are cleanup programs mentioned in the other answers. Safest course: Back up your data and re-install the operating system after formatting the disk using a bootable CD.