Whenever I get on this site, I get a page that says I have private messages. When I click on the envelope, I get directed to a Best Mail site that wants me to download Inbox Ace to receive these messages. Is this part of the forum? If not, I need to check my security.
No it is not a part of the forum. You don't need any third party software to read messages of this forum. You may want to scan your PC for malware/spyware programs.
That is an annoying ad, and does not say "advertisement" like most Google ads. Google should ban ads like that and "Upgrade Flash Player" scams/malware. If you see a banner ad saying you need Flash don't click on it, it's a bad one. I spend a lot of time fixing other people's computers from those. Only download updates from the official websites, not ad links.