I've never seen an ad while signing in, but then again I've got this machine set to revenge level settings because of all of the the unwanted flashy, blinky crap that was pushed over my dial up connection back in the day.
Just saw this for the first time. I am using Firefox and I did a full scan (including rootkit); no malicious items found. Here is a screen shot:
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Failing to plan is planning to fail!!!
You might want to try Addblock Plus, https://adblockplus.org. It is an open source free addon for most browsers (Firefox, Chrome, IE, Opera, Safari and Droid). The home page will detect your browser and automatic take you to the correct download for your browser. I installed it on my Firefox at home because I started getting ads popping up in front of the start of youtube videos and by the time I could close them, I missed part of the video.
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