Yes, you can install Linux in a dual boot configuration that gives you a choice at startup as to which way you want to go. I'm posting from a system running SuSE Linux as my default boot with Windows XP as an option.
This system serves four users - my wife, myself and two kids. We all use Linux for general computing and ALWAYS for web browsing. I finally made the switch when the malware maintenance and security issues became so bad under Windows that it surpassed the inconvenience of installing and "learning" new operating system.
I still use Windows once a week to prepare a PowerPoint set for a network of churches but that's a 30 minute deal once a week. I'll forego even that once I get the time to test format compatability of running MSOffice under Linux - my schedule right now is a bit too tight to organize a network-wide test. Once that's done, I plan on only booting to Windows once every few months - to upgrade virus and security programs!
If you're interested in trying Linux, you can do so without installing it at all. If you have a CD or DVD burner, you can download a "LiveCD" image which will allow you to boot and run Linux directly from the CD without anything being installed to your HD. SuSE provides both free DVD and CD iso images. Others do as well, notably Knoppix (which is designed to run entirely from the CD). Let me know if you need details.
If you like what you see, SuSE can be purchased as a boxed set with excellent documentation or actually installed directly over the internet for free (the route I took for the latest version).
I, for one, an convinced. I won't be going back to Windows.
Off the spyware/virus issue, but I am sure Open Office has a linux version, and should handle presentations OK.
Also, an advantage to Firefox and many other 3rd party browsers is the ability to open tabs, VERY useful in FR browsing!