Potential Ways to Solve Windows Vista Viruses and other Malicous Softwares Problem

On microsoft homepage (http://windows.microsoft.com/is-is/windows-vista/what-is-a-guest-account) I read the following about guest accounts (what you sign in to after Windows Vista booted):

People using the guest account can't install software or hardware, change settings, or create a password.

My idea is that, if you cannot modify your Windows system, that mean that you cannot get viruses, malwares, etc either. I mean, viruses need to be "installed", and since you cannot install softwares, viruses cannot be installed either. At least in theory.

Do you think this true, or Windows is still prone to installed viruses in guest account.

The reason I ask that is because there are excellent free antivirus software, they always run in the background, which slows down computer, and decreases laptop battery life.

Also, another potential way to solve the virus issue is the use of virtual box. If you use a software in wmware virtualbox, you don't really install the software, and so it is kind of quarantined, isolated from the rest of the operating system.

I also use and surf the internet through Pendrive Apps, where my internet browser (with all bookmarks, settings, etc.) and other softwares are actually installed on the USB pendrive, not on the hard drive of the computer.

By the way, I mostly use linux (PCLinuxOS), which is way less prone to viruses. And of course a step forward in securing even linux is to always run it as Live CD, or rather as Live USB Stick.

So what do you think? Do you have other potential, innovative ways to prevent viruses, malware, troyans, keyloggers, and other malicious softwares? Please share them in the comment below!