Having been designing and building websites for years, the following comments...
1. Yes, you could get a trojan virus from a roll-over ad, by just rolling your pointer over the ad. The ad just has to be programmed correctly to execute a download and install on roll-over, and to defeat notifications to the user. However, proper settings on your browser should notify you if something is trying to install. Another good reason for keeping your system up-to-date.
2. I have seen rollover ads on this site. Mostly they just play Utube video ads for movies or TV shows. More evidence that you could get a virus from a rollover - if an ad can execute a play video function on a rollover, it can execute any command. See #1, above.
3. The admins have no control over what appears in the banner ads and bottom ads on this site, besides setting general guidelines. So it is conceivable that a virus could appear here contained in an ad.
4. However, since google and other ad providers supposedly clear all banner ads before releasing them, it is highly unlikely that a virus was passed via a banner or bottom ad here.
5. Get a good virus and spyware protection system on your browser and use it. It should throw up a warning and block any virus installations on your computer.