WinXPNewb, lets have a realistic look:
Don't worry about anti-virus unless you want to download stuff for passing on to Windows-users. Virus affects them, not us.
A firewall does make sense, even with Linux being unendlessly safer by design. Its all about being on the safe side, although you probably won't ever need it. I have never heard of a Linux user really needing it on a desktop.
But consider this: Even in Linux it is good practise to have a copy of your own files (letters, bills...) outside of your box. Should your box ever get messed up, get stolen, blow up or get attacked by virus, your stuff is not lost. Remember how quick and easy it is to install Mint? Reinstall is even quicker, and you just add you own files and you are back with a running system in no time. Don't worry, be happy with Linux. But thats just my personal opinion after four years of Linux at home, and almost 15 or so of Windoze at work.