There is an exhausting long thread at WebmasterWorld that I have been tracking since it started. The thread discusses that some Google AdSense ads, specifically the new video ads, may be prompting 3rd party software, like Windows Media player or Apple QuickTime, to install on your computer.
The other night, AdSenseAdvisor confirmed that this was indeed happening;
Thanks for bringing this to our attention. We encountered some technical issues that caused some visitors using IE to see download prompts for third-party add-ons or plug-ins. Our engineers resolved the issue right away, so ad serving is now back to normal.
Don't worry -- all clicks and impressions have been recorded as usual. We're sorry for any inconvenience, and grateful for your patience during this time.
Forum discussion at WebmasterWorld.