I'm thinking that since your local systems scan clean of malware, then the possibility of your internet router being abused is a higher probability suspect (or maybe your cable modem, but more likely the router). As I said before, there's been a lot of discussion on the security lists about home routers and home cable modems being compromised, some by a single malicious packet sent to them. I hope you've checked the firmware version on your router, updated it if needed and changed admin password on it (and on your WiFi).