It has not been assigned to a Kingdom in the standard taxonomic classification system. Currently the taxonomy of viruses is less defined that of other and living organisms. Since viruses are really not living organisms, they do not have a Kingdom assigned.
Ordinarily (except for viruses) the hierarchy is:
Life
Domain
Kingdom
Phylum
Class
Order
Family
Genus
Species
For viruses the hierarchy and classification usually begins at the Family level and continues down to the Serotype or subtype and strain, however, there are many different methods of classification with some more specific and some much less specific.
A-H1N1/09 Pandemic Influenza "Swine Flu of 2009" (an RNA virus genome)
Kingdom: Unassigned
Phylum: Unassigned
Class: Unassigned
Virus Groups (I - Vl): in some classification systems Influenza viruses fall in Group V, in other systems they are unassigned
Order: -virales or unassigned
Family: Viridae/Orthomyxoviridae
Genus: Influenzavirus A
Species: Influenza A virus
Serotype/Subtype: H1N1
Strain: A-H1N1/09-like virus