As Richard noted (22 Dec), "MSE just went to 2.X".
There now is one exe for all 32-bit Windows (XP, Vista, 7), and one exe for 64-bit Windows (Vista, 7; not XP). The Download Now
panel doesn't make this clear.
For a comparison of several security products (4 free, 7 paid) see this CNET
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:22.214.171.124) Gecko/20101203 Firefox/3.6.13 (.NET CLR 3.5.30729)