Martha's Mum said (in the 'People knocking on the door asking for a roof survey' thread):

This is more interesting than I would have guessed. The Oxford English Dictionary gives the plural 'viruses', which is what I would naturally use. It comes from the Latin word 'virus', which means 'slimy liquid' and I have always assumed that word is masculine and fourth declension (so plural 'virus', with a long 'u') rather than second (which would normally give 'viri' as the plural if it were masculine like almost all second declension nouns)

However it appears (see this Wikipaedia article) that 'virus' in Latin is an uncountable 'mass noun' and therefore doesn't have a plural. Scholars also dispute whether it is in fact second or fourth declension. In any case, it appears that it is neuter rather than masculine and therefore perhaps one of the very rare second declension neuter nouns. In that case one might form a plural 'vira' for the word in its modern sense. 'Viri' would in any case be highly irregular, although according to Wikipaedia it was one time quite commonly used in computing circles.

'Viruses' is probably safest!