I hate sony with a vengence, AVCH is a pita
but for your old Handycam DCR-HC21 I do not think that USB will work with linux.
That old handycam as far as I can make out, saves in (some sort of) DVD format. It might not be full DVD, the manual does not say. From the manual, outputs to composite video or S-Video
see page from manual http://i.imgur.com/hVU17pO.jpg
So save to a DVD recorder on DVD disks (write once or re-recordable)
You will get the video in xxxx.vob files and copy to your computer
Use Avidemux or OpenShot for editing and DeVeDe or DVDstyler to author into your DVD masterpiece.
Splash out on a firewire card and go straight into your computer.