An organism, according to the Oxford English Dictionary is:
"[an] individual animal, plant, or single-celled life form"[1]" and this excludes viruses.

Plants are identified by having cellulose cell walls and making their own food supply by the use of chlorophyll. So bacteria do not qualify as plants.

Viruses can only multiply within the cells of other organisms, and thus cannot have a separate existance. They are in a class of their own.

Animal because they do not make their own food.

The difference between animals and plants is that animals can move around, plants cannot. Therefore, as they are not rooted to one spot, I believe that bacteria and viruses are closer to animals than plants.

Perhaps it ought to be said that viruses are not really like animals or plants since they are not technically 'alive' i.e. they do not have a metabolism. They are just a bundle of DNA wrapped up in protein. So in terms of scale they are closer to the baccies, but in terms of form, behaviour and evolution - they're a long way off from both.

Scientifically viruses and Bacteria are neither plants nor animals.

Lets tackly viruses first - simply they are just macromolecules of DNA or RNA surrounded by a protein shell... they dont have a cytoplasm, way of manufacturing proteins, cellular respiration and so on common to other living things. Earlier they were classes as non-living but discoveries of new viruses which have some abilities of other living cells just means they have been classed in a group of their own for now.

If we take all other living things they are classed divided into 2 (Prokaryotes and Eukaryotes) basically Eukaryotes have a nucleus and membrane bound organelles in their cells while prokaryotes don't.
Under prokaryotes come the kingdoms of Archea and Monera (Bacteria come under this kingdom)
Under Eukaryotes come the kingdoms of Protisa (things like amoeba, algea), Fungi, Plantae (plants) and Animalia (animals).
So bacteria being on a class of their own cant realy be close to either they are as different from plants as they are from animals.

more reading
http://waynesword.palomar.edu/trfeb98.htm
http://biology.about.com/od/evolution/a/aa091004a.htm
http://en.wikipedia.org/wiki/Kingdom_(biology)