You can purchase standalone hardware that can sniff your network data for viruses, but they aren't cheap or consumer oriented. Your best bet is safe browsing practices and vigilance with anti-virus and malware scans on a regular basis. Also, make sure to back up your important files often!
Hmmm... so I guess what I'd be interested in is a piece of hardware that would constantly scan for viruses and maleware, but wouldn't use any (k, maybe very few) of my system resources. Any suggestions?