Are there known Windows viruses which are capable of ARP spoofing? If so, how widespread are they?
ARP spoofing is tricky to do blindly. If the virus just starts ARP spoofing, there is no way to tell just how many computers might send traffic to the infected host, and DoS the infected host. So, most ARP spoofing attacks choose a victim to attack. A blind virus is not going to be able to do this intelligently. It is also difficult to blindly extract value from this act. So, no, it is not likely that one will get a virus that ARP spoofs.
But, if the "virus" is actually a RAT, then it gives control of the victim to a user who can make intelligent decisions. But I'm not sure if this falls into your definition of "virus". There are many, many RATs of many different kinds that can infect every kind of computer. So, in this definition, there are many and they are very widespread.