I suspect that pop-up has to do with the latest update of the Java Runtime Environment (6.20) which came out not many weeks ago (assuming that you've let it automatically update itself, as it normally does). I say that because I've started getting the same pop-up on my internet banking website, which didn't happen before.
You can try to manually uninstall Java and install the previous version of the JRE instead (6 Update 19), which you can download from the
Java archive.