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Recently a co-worker viewed our site from a different location. He said that AVG came up and said the site was not secure and blocked the site. However, when I run the site through AVG it says the site has no malware and no potential threats.
We do not sell products or collect information. So we do not have a secure layer on the site.
It's possible he tried to surf to your site as https://yoursite.com instead of http://yoursite.com and AVG told him the security certificate is not valid and blocked the site. Browsers usually handle that, but AVG may have handled it in this case
If your site does not have a Security Certificate, browsers and anti-virus will warn you that an SSL error has occurred when you surf there using https.
Some types of malware that creep into sites are triggered by "referrer" eg; using a bookmark, or going directly to site comes up clean, however when the referrer is a search engine like Google or Bing, the malware becomes active, and detectable. The old "pharmahack" worked this way.
When in doubt, FTP a copy of your sites files and scan them locally. You won't risk infecting yourself unless you open then with a browser. You can also search for tell tale code snippets, like PHP's "eval(" function is often following by encryption functions to hide nasty stuff. Of course there are legitimate uses for "eval(" so that alone does not indicate infection.
Maybe check the website on http://sucuri.net or similar which will show you the blacklist status on some of the most popular monitoring services such as Google, Norton and SiteAdvisor etc.
If you haven't already, set up a Google Webmaster Tools account including a valid email address for the website and Webmaster Tools will notify you when malware is detected.
