The experience is crippled by Apple's lack of effort towards drivers, particularly in their laptops. (e.g. battery life hours shorter than it should be, touchpad not as good as Synaptics despite having the potential, etc.)
And a mac running windows is just as likely to get viruses as any other Windows system.
The same amount of money spent on a prebuilt PC will likely be equal in reliability, and more so if you include the ability to replace parts yourself.
A self-built PC will be easily more reliable if you pick the correct parts
Macs are expensive because Apple can set whatever price they like on what is really mediocre hardware. Paying for the design/'build quality' makes no sense when companies like Sony can easily match their engineering prowess on high end portability/business focused laptops at a lower price point (e.g. Vaio Z series, Vaio Pro series).
The NT kernel/system is probably just as fundamentally secure, all things considered, except it's just commonly used in a way that makes it inherently insecure (e.g. UAC disabled, admin account for normal use, etc.)
While IE is certainly tightly integrated, it hasn't been a major source of security issues since Windows ME, as long as you keep up with windows updates