possible bug?

[ComicChick]

There are some sites I'm having difficulty viewing.

When I check my Yahoo! mail, I get a message saying that another program is preventing me from viewing it.
At first I get a login error thingy, and then I get this:
"Ooops. Yahoo! Mail can't load
A non-Yahoo! software program on your computer has modified your Mail page. Hence, it can't launch.

You might try going into your firewall settings, and disabling "Ad blocking". "
And then I get a "Click here to try again" which usually works ok.

Is there anything I can do, or any programs I can run to find out what's causing the problem?

In case it matters I've been using IE7 and Opera.
I also have Clamwin and Norton