I currently live in a building with around 40-50 other tenants (mostly students) and we all share one connection to the Internet using one of 3 wifi routers that are spread throughout the building. Yesterday, our landlord got a message from the ISP stating that there is "a virus" on the network, and that there have been complaints about lots of spam going out to the Internet from our IP address. If the problem does not get resolved in one week, Internet will be cut off from us. He also said left a message to all of us to scan our systems with AVs and to send him scan logs but if the situation is that bad already, I believe that his approach will be of little help since AVs on these machines already failed to identify malware on the system.. And in case they don't even have any kind of protection, trying to install it now that they are already infected also won't be very useful.
So, I thought of offering my help to the landlord to help in locating machines that are infected and that send all this garbage out to the Internet. I don't have access to our network "infrastructure" at the moment, but I might be able to get it if I offer them my help.
My question is - what could I do to help him detect and locate the infected machines? Are there any tools out there that are made for the purpose of finding infected machines on the network? Should I maybe set up an IDS on my system and then look at what is trying to attack me? I know that I could use a vulnerability scanner to locate some stuff like Conficker for example.. Maybe Nmap? Other ideas?
I would prefer an approach where I could detect these machines from my own computer since our network is made from small home wifi routers and does not have the capability to do anything fancy. I also don't want (and don't have the time) to go to each and every tenant in our building to clean their computers from malware manually. I just want to find out who is infected so he could get taken off the network until he gets his machine cleaned.
One more question - if the solution involves scanning computers on the network, do you think I should ask for a written permission from our landlord that I am allowed to do the scanning?
Any ideas are welcome. Thank you.
edit: Regardless of will I be allowed to do this or not, I would be happy to see some possible solutions to this problem.