No it isn't.
In Latin or in English, just because a word ends "-us" doesn't mean it's pluralised to "-i". See
http://www.perl.com/language/misc/virus.html.
And to be particularly pedantic, as that page points out, "virii" is especially wrong - if you were assuming that "virus" was pluralised in the same manner as "radius", then the plural would be "viri"
