ME-tan is clumsy because her OS is unstable. The exclamation mark represents the Active Desktop error you'd see if you had a .jpg as a background since MS was nuts enough to use Internet Explorer to handle desktop backgrounds (they wanted you to be able to put a whole ass webpage as your desktop), and that was a complicated enough component that it could have been a reason for the crash. Since no one really used active desktop to do anything but display non-bmp backgrounds, it never was the issue, but on an unsafe shutdown, you'd see the big triangle.
ME-tan is cute, but Windows ME sucked ass and I was unbearably glad to install XP back in the day, even if RAM was a little tight with 128MB in the machine. Still better than having AOL lock up the whole computer under ME.

XP-tan has big tits because her OS needs a lot of RAM.
95-tan wears a traditional outfit because 95 is old.
3.1-tan is a lolibaba because 3.1 is ancient, but Windows 3.1 is very light and small.

I don't know why 2k-tan wears the school swimsuit but it's very cute. She has tits due to RAM requirements, but not as big as XP's.