Os-tans are anime personifications of operating systems as girls. Another name for it them is OS-girls. It all started when some random guy in Japan working at futaba channel thought Windows ME was like a tempermental troublesome girl who did all kinds of stupid things that lacked common sense.
A phenomenon started on futaba-channel where people made personifications of the operating systems out there. Find a long and better description at http://en.wikipedia.org/wiki/Os-tan
Geek1: XP-tan rocks so hard!
Geek2: All of the OS-tan's are so cute~ X3
Geek3: Yeah right.....