possible virus

My Yahoo mail account just got hijacked. Without me doing anything, the persons in my address book receive an email from my email address asking them to click on a link which turns out to be a Viagara ad, hopefully not worse. I had read an old Macworld article about malware. In my Preference folder, I click on Ethernet (which I use) --> Advanced --> DNS tab. There in the DNS Servers window, I see in grey "10.0.1.1". In the Search Domains: window, I see "myhome.westell.com". Can someone tell me if my system is infected? Thanks.
Tom