Possible Virus?

Okay so about 2 weeks ago we got an email saying that someone had bought 3 kindles on our amazon and after chatting with customer service we found that they changed the email on the account ours was _____@comcast.net and they changed it to some random .net site i have never heard of and dont remember and when i went there it said not a valid site.
About 8 days later another email that we use on the same computer but not in mail (In the browser.) was hacked from Bulgaria.
Are these incidents relateded to eachother? Should I buy an anti-virus?
I have a mac desktop with 10.6.8 and intego anti-virus that expired in 2010 that i never renewed.