Because it would have to come through a vector that I haven't completely walled off, and those are exceedingly rare (like the WMF thing ages ago). I keep up on vulnerabilities, and I'd do a paranoia reinstall if I suspected I were affected by one.

It would be one of these:

* Noscript exploit

* Simultaneous Flashblock + Flash exploit

* Image-based exploit

* Renderer exploit

And those are extremely rare. My software stack is also pretty stable, and it's all from trustworthy vendors/projects.

You are clearly the typical XP+IE6 user being targeted by this campaign.