If you installed Photoshop CS from scratch, not the case with an upgrade, you probably noticed something very disturbing. PSD's don't have thumbnail previews in Windows, not even in Photoshop's Open dialog! It turns out Adobe left out a DLL called psicon.dll do to crashes on some peoples systems. Well I for one find it extremely annoying not to have thumbnails in Windows, fortunately the solution is simple. Download psicon.dll and place it in C:/Program Files/Common Files/Adobe/Shell. That's it, they are instantly back.
Thanks to notestips.com and dlldump.com.
Want thumbnails of Canon Raw files? Head here.