Today, I went to 9down.com, which seems to be a fairly well-known and legitimate site; I didn't download anything from there, anyway. However, about 3 seconds after navigating there, my browser was directed to some site advertising for Personal Antivirus (Which I know is a rogue). It was called clean-your-pc.info, I believe. A second after I was brought there (I didn't click anything) a message comes up claiming that my computer had a virus and that I need to download Personal Antiware. Without doing anything on the page or clicking the message, I closed Firefox, disabled my internet connection (paranoid, I know. Just trying to be careful. ><) and ran an MBAM scan. Nothing came up, so it seems I didn't Personal Antivirus a chance to install itself. However, I'm still a bit unsettled as to why I was directed to PA's site, since pre-existing trojans usually cause that. What should I do? Might I have a deeply embedded trojan or rootkit or something that won't show up on MBAM? Thanks.
[edit: Clarified subject.]