It has been reported that Google's Adwords network has been used to try and direct people to malicious websites.
Cyber attackers' latest attempt to compromise your machine doesn't involve using the adverts that appear above and to the side of search results when you're using Google (if you're not aware, up to three of the first results returned when you search for something are paid-for adverts. These are marked with "Ad" beside them, and are not necessarily the best match for what you searched for).
Instead, this attack used the "adsense" banners that you see on many legitimate websites. People or companies can place these banners on their websites to make money from Google when people click through to the ad. However, in this case, one of these adverts had been constructed in such as way that once loaded (i.e. no clicks required), automatically forwarded people from the legitimate site off to a malicious one.
Fortunately, Google appear to have squashed the bug in the adwords code that allowed this to occur. However, this highlights the necessity to remain vigilent at all times for anything unexpected that happens while browsing the internet.