Ubuntu doesn't come with antivirus, it depends on user permissions, and security updates to keep itself from becoming infected. Additionally most antivirus tools available for linux DON'T scan for linux viruses, they only scan for Windows based viruses. The approach of updates and permissions makes it difficult for a virus to infect a system, and very difficult to determine if a linux system has files on it that contain viral code. It's one of Ubuntu's biggest drawbacks in my eyes, because many people end up unconciously helping to spread viral code to other systems. It should however be noted that there aren't very many viruses for linux (as far as we know).
Comodo Antivirus For Linux is one of the best free antivirus products I've found for Ubuntu, it comes in 64bit and 32bit, but only scans for Windows viruses. It also doesn't work on all versions of Ubuntu due to driver problems.
ClamAV is a simple manual scan only antivirus for 32bit and 64bit, again it only detects
Personally I would suggest running a Windows manual scan antivirus product in wine since you will have a better selection, and either way you only detect windows viruses.