problems with DNS requests

One of the workstations at my client's sends huge amount of DNS
requests to one IP address based in Russia. I found that by checking
the traffic on local network as users started complaining at slow
network and Internet connection. I run Malwarebytes and Panda on that
PC. Malwarebytes detected some threats and removed them. Panda found
some virus and quarantined it. Just when I thought I sorted that out
it started again. Malwarebytes is not detecting anything now nor or is
Panda. How do I deal with this bug? Considering reinstallation but
maybe someone could suggest a better solution?