I have posted before about Gator downloads being prompted on this site, to be told that you don't deal with Gator at all. I understand that, but I think one of the banner ad agencies you use does. I repeatedly get offered gator downloads on this site, and it appears others do to.
I have repeatedly scanned my own hard disk for spyware, and even recently rebuilt yet still get them. Might be worth checking with whoever supplies your banner ads...
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