I voted for Linux Distros. These are very solid OSes. They are generally difficult to spam with malware and/or viruses (if your permissions are properly set) and are simple and minimalist enough to do what you need to do quickly and efficiently. They are my OS of choice for computer and internet related tasks.
However, I do not even run a Linux Distro on my computer atm (i used to dual boot on my old computer) because windows 7 brings the same functionality with the added bonus of solid support for all my games and, most importantly, DirectX.
For anyone who is a non-gamer, I hugely reccomend a linux distro. Specifically, Ubuntu is a good balance of being user-friendly and pretty, but at the same time simple and fast.
Addressing topics from this thread:
Generally, people are treating viruses as something that just is. However, remember that someone had to write the virus. Not only that, but they had to have a purpose. Sometimes the purpose of the virus is to infiltrate computers and gather data. Other times it's simply to destroy someone's computer because the creator is sadistic or wants to show that they can. Either way, viruses are aimed at an anonymous user, and the creator has a motive to distribute it.
Now look at it this way. If you wanted to steal (gather data) or destroy (disable a computer) using something that can reproduce itself infinitely, what would you target?
Obviously one would target the largest possible demographic. In this case, that demographic is windows users.
It is not a surprise that the majority of viruses in the world exist in windows. This is not windows' fault; it is simply the biggest target because of its large user base.
If one wrote a virus for linux (which is extremely difficult, but let's say one does). The expected number of infected computers is dramatically less than if the virus were written for windows.
So please do not address the issue of "viruses" being a disadvantage of windows. If linux or mac OS were the largest OS demographic, they would be the OSes that are notorious for being "easily infected with a virus."