Carwiz made a good point about rootkits - quite a share of them can successfully hide from top antiviruses if scanned from inside infected computer. I would recommend to download a special livecd, burn a disc and scan infected computer from it. Good examples would be:
Microsoft Standalone System Sweeper Kaspersky Rescue Disk
Also, I would check logs of that router to see what was happening there.
Change admin password on it.
May be enable MAC Filtering on that router (and place all MAC addresses of home computers on whitelist), so anyone outside even with wifi password won't be able to use wifi.