I just struck strange bug, which really annoys me.
When I start new torrent task a window pops up where I can select specific files from torrent which I want to download by unchecking boxes near unwanted files.
Since I moved to x64 system and installed 64-bit version of BitComet it just ignores this checkboxes and starts downloading all files from torrent in any case.
However if I choose “Download later” and then uncheck some files from “Files” tab and then start downloading it works properly.
I got Windows 7 Professional x64 system, if this information could be any help.