I'd just add to the Linux bit, ,,, Ubuntu 12.04 is being released in April and is a Long Term Support version. They usually release a LTS every 2 yrs.
I use Ubuntu, probably the best thing about it is the very fast boot time (from 0 to blazing the internet within 30 seconds) Takes a bit of getting used to though. Installing most software is fairly straight forward . You can run windows programs if you install WINE or Play On Linux.
Email is easy to set up. Using the terminal (very similar to the run command in windows) is a bit harder, but there are plenty of support forums for Linux users. There are no viruses out there specifically aimed at Linux systems (although you can unwittingly pass Win viruses on to Win users.
You can download the OS onto a usb stick and use that to run your computer (you have to go into the bios and select usb device first in the boot order though) And you can make the usb stick persistent, so that's your OS and this leaves your HDD for storage of personal files etc.
When Linux goes wrong though, it will go wrong BIG TIME, but a quick re-install usually fixes it.
As daozen says. You can make your computer dual boot, so you select which OS loads on startup, Win or Linux.