Wiki to the rescue..
In the English language, the standard plural of virus is viruses. This is the most frequently occurring form of the plural, both when referring to a biological virus and when referring to a computer virus.
The less frequent variations viri and virii are virtually unknown in edited prose, and no major dictionary recognizes them as alternative forms. Their occurrence can be variously attributed to hypercorrection formed by analogy to Latin plurals such as radii; idiosyncratic use as jargon among a group, such as computer hackers; the incorrect assumption that the word is of Greek origin, requiring an -i plural; and deliberate word play, such as on BBSs.
To complicate matters further, viri is already used in Latin as the plural of vir, meaning "man" (thus making viri mean "men").