For me , a MAC is useless , you can get a PC with x10 better performance with the same price than a MAC.
PS: You can have a Mac OS X on a PC
No doubt about it; PC > MAC. Even Windows 98 > MAC in speed because you can tweak every setting it has to offer for a speed increases + custom computer parts like video cards, processors, motherboards, etc.
PC's are also damn inexpensive. You can get a used $40-$50 'working' laptop (Windows XP, 98, ME, 2000, possibly Vista) that is used on eBay. Of course it will be maybe B or C grade, maybe won't hold a long charge (Comes with AC adapter so it don't really matter) but, if your using it for browsing, checking email, light gaming, watching videos, it's perfect for you.
I don't see working MAC books for $40 on eBay.
And the thing about viruses,
I have never gotten a virus on my computer before and I don't even use Anti-Virus protection (Notice how I said I and not my family).
You can't get a virus just for browsing through the internet, it has to attach itself to your computer (With your permission of course). People get viruses because they go to some site that prompts them to download something and they say 'okay' lets do that. Some people still believe in key generators, or hacks for sites that don't even work!
Just know what you are downloading and you will be fine.