There is such a thing called 'drive-by downloads'. Whether or not these are viruses I don't know, very few of my computers have ever had malicious programs installed without my knowledge. However, if you have no protection, it stands to reason that you are vulnerable to these. I don't know all the details, I am careful where I go on the Internet.
I believe some programs from McAfee include a feature called 'SiteAdvisor'. I haven't used McAfee in quite a long time, but I believe it adds alerts in search engines (such as Google) on websites that are known to be malicious.
I have also looked SiteAdvisor up and found that it is a free download:
https
/www.siteadvisor.com/final/index.html
Hope I helped.