Assuming you have a codec installed that allows you to view AVCHD files in Windows Media Player (such as CoreAVC), here's how you can get those videos' thumbnails into the Explorer thumbnail view. I wholly admit I stole this info from some forum somewhere, but most of my searches turned up some lamer's warning about using regedit, and a refusal to post the fix because of it. So I'm going to post it again in a concise article.
Don't freak out. Regedit isn't that damaging. In fact in the 12+ years I've been working with Windows, I don't think I've ever seen a computer explode due to a change in regedit, especially when you follow directions.
Anyhow, open Notepad and copy in the following:
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\.mts\ShellEx\{BB2E617C-0920-11D1-9A0B-00C04FC2D6C1}]
@="{c5a40261-cd64-4ccf-84cb-c394da41d590}"[HKEY_CLASSES_ROOT\.m2t\ShellEx\{BB2E617C-0920-11D1-9A0B-00C04FC2D6C1}]
@="{c5a40261-cd64-4ccf-84cb-c394da41d590}"[HKEY_CLASSES_ROOT\.m2ts\ShellEx\{BB2E617C-0920-11D1-9A0B-00C04FC2D6C1}]
@="{c5a40261-cd64-4ccf-84cb-c394da41d590}"
Save this as "AVCHD Thumbnails.reg" and run it. Accept the merge. You may or may not need to reboot to apply the change. Hit F5 in your Explorer window to check.