I would only use Windows if I was trying to break my computer.

Ubuntu is lacking when it comes to games. I use my Mac for gaming and there are still a few companies that are clinging to DirectX and don't make Mac versions of their games. Windows wins there.

Not only viruses, most people are fairly well protected against them. It's spyware that normally slows down your computer. Linux is immune to that as well. Another advantage to Linux is that the way that it's designed, it has a shelf life that is much longer than PCs.

Windows uses something called a registry. It stores lots of stuff about programs and files that it doesn't need. If you delete the file, the registry entry remains so it gets really cluttered and fragmented. Windows has no way of fixing that so with time it seriously slows the computer down.

Windows is included in the price. My dad just got a brand new laptop, it was $100 cheaper than a Windows equivalent because it was preinstalled with Ubuntu.