In what universe is it acceptable for vendor A to modify vendor B's software on User C's (i.e. my) computer? To modify it at all, let alone with security-impacting ramifications?

Earth to Microsoft: drive-by downloads are among the worst of vulnerabilities. They must be avoided at all costs. And the way to avoid them is not to be more careful when writing and installing unnecessary little browser plug-ins. The way to avoid them is not to install unnecessary little browser plug-ins in the first place. (And if you simply must install unnecessary little browser plug-ins, do it with your own grotty browser, not the non-Microsoft one I installed specifically to avoid all the security concerns of yours.)

Sheesh.

I won't mod you as flamebait (no mod points today), but I will respond to this bit:

Macs crash just like a Windows computer. Macs experience hardware issues. Macs, if you use them heavily, need regular maintenance to keep them running smoothly.

You're one for three, in my experience. Hardware issues: yeah, I've had a few. But my Mac just never crashes. And I have no idea what you're talking about when you say that "heavy usage" implies "regular maintenance". My Mac runs smoothly all the time, and the only "maintenance" I do is backing it up regularly.