iPad wants an mp4 file, so you need something to convert AVCHD to mp4... as far as I know, there aren't many free tools to do this on Windows. You can get it to work using ffmpeg, which is free, but not necessarily straight forward to use (it's nominally a command line program, but there are some GUI front ends to it). Funny that there are many great free programs on the Mac to do this (i.e. iMovie, Handbrake), and Windows is supposed to have all the software...
But if you do want to shell out the $$, Sony Vegas would do it, and many other video editing programs would too.