Well, I wrote a Miku one so I'll write an XP-tan one.
When I was entering college, I bought a computer. XP taught me everything I know about computers, and she always had a smile on her face. She was kinda chubby and would eat up a lot, but I like that in an OS. She'd always tell me about her problems and I was happy to listen.
Most people have dumped their XP in favour of that totally transparent, shallow Vista-tan. But I'll always love XP-tan.