This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
Pineapple Software have produced a program called !Killer, which is the definitive means of checking for or killing viruses. There are a few other commercial solutions, see the next question for details on how to obtain them.
There are also a few PD programs around, but these tend to detect only a small subset of the viruses in circulation. !VKiller used to be OK, but it is no longer maintained, is now seriously out of date and fails to work under RISC OS 3. Out of the PD virus utilities the current best is Kiwisoft's !Slayer. This detects most known viruses and removes quite a few as well and serves as a good secondary defence if Killer is unavailable to you. Scanner should be available on various FTP/Email servers. I have als o produced my own virus killer called !VZap.