You should take a look on Ubuntu. No viruses, great stability, great security, and really easy to use.
http://www.ubuntu.com/The great news is that it's FREE (like free of charges AND opensource), it works on any PC (even the older ones), from any brand, and you can still use a lot of windows programs too (using WINE software).
All of the apps you like (chrome, firefox, thunderbird, open office) are available. Most of the apps are free and are downloadable from inside the system, using some kind of secured official "market" software. No more risk of virus, bloatware, or spyware like when you were downloading things directly from internet.
If you don't find an app you had on windows or mac, 99% of the time you'll find some equivalent (or better). A lot of apps and games from windows can work too using WINE (available in the "market" app).
You don't need to find the correct drivers for your graphics card, sound card, keyboard or printer. Everything is installed automatically, and ready to use.
Flash, video formats, music formats, everything is also installed. No need to install codecs from suspicious sources.
If you"re a gamer, STEAM is available officially on linux now. STEAM OS (new standalone version of steam) is based on a similar core you find in UBuntu.
Personaly, I switched to UBuntu in 2006. I never went back on windows or mac. Now I install it on friends and family computers when they have problems, instead of other systems. Once they lost their bad habits, they never call me again for computer problems.
The great thing is that once it's installed, it works, and a few years after, IT STILL WORKS like the first day. That's because of the OS architecture and filesystem (to make it simple). Because of the way the system is made, you virtually (if you use the system the way it has to be) cannot brake anything, or have viruses.
Last but not least, you can TRY it using the live DVD. Download ubuntu, burn it on a DVD (or install it on a usb stick, it's explained how on the official website), and boot your PC on this dvd to try it without installing it, to see see if you like it first.