If your virus protection has already blocked the ad there's no need to run away.
Right-click on the ad and select copy link location (or copy link), then paste it into Notepad or Word or something. I like notepad because it doesn't show it as a clickable link, just as text.
One you paste the URL, look toward the end. (In Notepad turn on WordWrap to make it easier on yourself). The URL for the target site is listed.
For instance, I have a PRWeb / Vocus ad on this post right now. I right-clicked on it and copied the link to Notepad:
http://ad.doubleclick. net/click%3Bh%3Dv8/3b9b/2/0/%2a/y%3B236080528%3B0-0%3B0%3B59384825%3B3454-728/90%3B40601522/40619309/1%3B%3B%7Esscs%3D%3f
http://googleads.g.doubleclick.net/a...344&adurl=http://service.prweb.com/go/psg_dcobrand_ba/%3Futm_source%3Dgoogle%26utm_medium%3Dcpc%26utm_ca mpaign%3Dpsg_dcobrand_ba&nm=3
Near the bottom you can see
http://service.prweb.com is the target site (everything after that just tells that site where you came from).
Probably clear as mud but I thought I'd share, in case it helps the next time. :)