For starters, linux (unix in general) is more secure by design unless you are the village idiot who insist on logging into your Ubuntu setup as root (you actually need to hack this as Ubuntu by default prevents users from logging is as the root user). What few viruses/worms that do exist can only affect the user as per his/her access rights to the system.
However (I did say I've changed my opinion on this), Ubuntu users do receive a lot of files from other users who (gasp!) use Windoze (and Macs) and these platforms do have viruses/worms. What we do not want to be is a carrier for those malware. So installing an anti-virus should be done only for the occasional health scan and to maybe scan new files as you get them.
I have Googled around for a decent, free, linux based anti-virus and have found the following:
- ClamAV (URL http://www.clamav.net/lang/en/) or check with Synaptics
- avast! (URL http://www.avast.com/linux-home-edition)
- AVG (URL http://free.avg.com/gb-en/download.prd-afl)
- F-Prot (URL http://www.f-prot.com/products/home_use/linux/)
You can also check the Ubuntu page on anti-viruses here.