OS-tans are anime style characters, usually female, that are dressed to represent specific operating systems.
http://upload.wikimedia.org/wikipedia/en/b/b6/Ostans.jpgThey have become SO popular that other things have started getting one as well. Like webbrowsers, hardware, and antivirus programs.
(note that the only male there represents norton antivirus)
But do be careful when doing a search for them.
Hentai versions are usually not far behind.
And one can even find cosplayers dressed up like that.
(even in the more revealing outfits) ^.^