If it only happens when you log into the marketplace, I'd be more worried they're trying to steal your credit card info rather than your downloads. Sounds more like you guys all got hit with a spyware virus that is triggered by keywords that may appear in the taskbar of your browser... like 'market'. Try going somewhere else that also has the word 'market' in the URL, and see if it happens again. That should narrow it down to being possible spyware, or something fishy with this marketplace in particular.

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Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.