Windows 7: Strategies to avoid virus spread in a LAN?

Long Story: My friend is coming over to get his pc fixed because it just happens to be so that his internet was shut off because he forgot to pay and he isn't responsible for the bill in his household. He always has a trojan or two in his pc and because of that I'd like to somehow "quarantine" him in the LAN so that his trojans viruses etc. can't infect other pc's in the network. I was thinking of attaching another router to the first router, so that it makes a new internet connection, and he wont connect to my main wireless network, so that he cant infect my machine through lan? TLDR: I want to make it so that it is as if he was on a far away modem in another house, so that he cannot infect my machine through lan, but still having him access the internet through my modem, by possibly adding a second router that is attached to the first router. Please give me better ideas on how to protect myself and if there are better ways to do this than the ones I mentioned. I don't want to sound demanding or bossy, but my friend is coming on the day the world will end, so I need help as soon as possible. Thank you in advance!

EDIT: The logic behind having 2 routers is this: because files of a pc can be accessed through a lan, having a second router connected to the main router would create a separate network, so that files cannot be accessed by the machine in the second network, as if the lan networks were separated by the gap of the internet.

Is this how the logic goes?

Why not clean your friends computer of infection first and then plug it into your lan?

Im trying to get malwarebytes on his pc, and he cant wait for his internet to come back on for him to update malwarebytes. Additionally, as I said, he somehow cant use his computer without getting 10 trojans and worms, so it makes doing av scans everytime he comes to play lan games. (sorry, I didn't include this in the first post, but I am doing this to also give my main network protection when doing game lan parties.

Here are some good things to use.
Windows Defender Offline
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Download AdwCleaner 2.101 Free - Adware and toolbar remover - Softpedia
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Free Online Virus Scanner | ESET
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Before the computer leaves make sure it has MSE
http://windows.microsoft.com/en-US/w...tials-download

Thanks for those, I will use those. I am wondering if using 2 routers will provide protection though in lan parties, in which I cant really manaully scan everyones pc to make sure they dont have malware.

Even with two routers they will need to be on the same network in order to LAN game.

You can put malware bytes on a USB stick and run it from a USB drive on every machine.

You would need to set those routers up correctly or it will end up causing problems.
Connecting Additional Routers | Tech Support Forum

Routers are a good thing but not the total answer. Reading many post here I do believe Chev65 and others can guide you through setting up routers. I do believe starting at the root of the problem and that would be infected computers. Lan parties can be fun only if it is done in a secure fashion. Inviting a group of people to a party just to get their computers infected isn't fun. I would suggest having a set of security rules that apply to all who hook up to the Lan. Those that know how to be secure help those that don't know how to be secure. Of course any help you might need on security can be got here just by posting. We are here to help.

Malwarebytes is an anti-malware scanner it does not scan for viruses but is a good complement to any anti-virus installed.

I suggest you install avast or MSE for anti-virus and use Malwarebytes free version as it will not conflict with the anti-virus from the links below:

avast! Free Antivirus | Download Antivirus Protection Software

Microsoft Security Essentials - Microsoft Windows

Both are free.

Do multiple scans to ensure friends machine is clean, ensure a firewall is switched on the windows firewall is ok on vista/7 but not on XP.

Download a third party firewall for XP as the built in on only blocks incoming threats not outbound threats which may reside already on the computer.

Ensure windows update is set to automatic to ensure the computer is fully patched.

Secunia PSI will check for outdated programs that could be vunerable and update them.

Replace the host file as a precaution also.

Guide below on how to:

Restore Windows HOSTS File Back to Default

You can use a second router to seperate your friend from your network putting them in a different subnet allowing no communication between the two networks but no Lan parties for your friend if segregated as has already been pointed out.

Something i should have mentioned last night regarding the windows 7 firewall is outgoing protection is not enabled by default you will need to enable it apologies for that.

Please use the guide below to enable outgoing protection on the Windows 7 firewall:

Windows 7 Firewall Outbound Protection