Neuter nouns of the second declension don't generally have plurals that end in -i, but rather in -a, so "vira" would be equally possible.
It's also important to note that scholars don't actually know the proper plural of virus because they haven't really found one in extant literature.
Wikipedia has a longer discussion at http://en.wikipedia.org/wiki/Plural_of_virus#Virus
Re:Astalavista.com hacked, including details
by mistermann on Friday June 05, 2009 @03:03PM
(#643737)
Doo doo doo doo, doo doodoo do,
Doo doo doo doo, doo doodoo do,
Doo doo doo doo, doo doodoo do,
Doo, doodoodoo, doo doo doo doo doo....
A bit hard to communicate, but that's the keyboard cat playing all of you off.
"Virus" is however, in the second declension (virus -i n. "slime, poison, goo") with the oddity of being neutral while having a second declension -us ending which is normally a feature of masculine nouns. And indeed, its plural would be "viri".