there's hundreds of thousands of viruses any number of which could cause problems.
I recommend a full disc wipe, including the bootsectors, hidden sectors, etc.
Gparted may help in clearing the partitions.
You should also make sure and run a good AV, like avast or nod32 (worth it!)
Run linux on the computers you don't need windows on.
Use password authentication on all computers and require a password except from specific shares only if required.
Use windows firewall, it never really gets in the way. Except when trying to network between XP and 7, that's a real pain
To keep infection from spreading, use a DMZ (in the traditional sense, not the cheap router sense) to keep suspect machines seperate from the rest of the network. IPCop is good software for this. Such functionality is probably available in DD-WRT as well but I'm not sure.
Lock down the wifi with wpa2 or better. I know some devices (*cough* ds) can't connect that way, I recommend setting up a DMZ for that stuff too.
Obviously all this requires expertise, money and some research, and is not suitable for everbody. But it's what you need to do if you want to keep your network secure!
