AVG Just because it's free.....

[{ETN}Dragon{CLR}]

Cobalt is correct as well as grisu.
If you have the original windows XP on a clean install and connect to the internet without any protection. You can get the blaster and sasser infection easily without installing or downloading a thing.
As far as anti viruses go avg is good I have used it for years and it's free. Just so you know though no single anti-virus is perfect because no single anti-virus catches them all. If I were to buy I would probably get CA Labs or nod32 maybe Kaspersky. I like low resource anti-virus programs. I do like the new security essentials and have tested it. It does have good capture count and is very low resource but is still new and needs bugs fixed(a little aging).
Most people love AVG because it is free and is a descent anti virus couple this with a firewall and good internet habits and you can stay in good shape. I do recommend adding extra spyware protection as this is a bigger issue than virus infection these days. Such as super, spybot S&D, adaware but that is another subject.

[mapes]

If you have a real firewall and not a host based firewall and are careful about what you download and use webmail you should never need a anti virus

[Qicmee]

I wanted to reemphasize Mapes's point that by reading webmail online, email cannot infect your PC because the virus payload isn't downloaded.

With or without anitvirus, the risk is much lower when the PC isn't shared.

And for any potentially serious PC problem, timely backups are the boring-but-essential catastrophic insurance plan.

[{CLR} ObiTroy]

...

And for any potentially serious PC problem, timely backups are the boring-but-essential catastrophic insurance plan.

+1 It's also important to keep your backup media off site as well.

I started using Carbonite about a year ago. Previously I use to backup to DVD's and keep those at my parents house. This started to become a huge time consumer and hassle as my backups were approaching 50GB's. I usually did a full backup every six months, incrementals montly. I evaluated several of the 'online' backup services and settled on Carbonite. I can't really say that any of them were better then the other but Carbonite was the only one at the time that didn't have a limit on the size of the backup.