Why does Windows Explorer halt for a moment whenever I open a folder?

"See this article : How to prevent Windows Vista's 'green ribbon of death'.
It lists 4 reasons for this. Although none of them seems likely in your case, you might still have a look.

I would also suggest that you try this:
Open up Explorer.exe, go to Tools -> Folder Options -> View. Then go to the Advanced Settings and check “Always show icons, never thumbnails.”

You might also, just to be sure, do a chkdsk -r.

You can also use Autoruns to see if any funny processes start-up without your knowledge, and you can use Process Monitor to examine the disk activity while the green ribbon is boring you to death."

"Windows Vista (and Windows 7) read a folder and its contents every time you open it, or it detects a change. On slower drives (e.g. network, non-indexed), this can take a long time, and you'll see the progress bar.

I don't know how to turn this off - I normally use another file manager where possible. In Windows 7 it's a lot faster, and you can press the stop button at the right of the address bar, but in Vista, this might stop it from listing all the files."