Can you provide some sample file names? Does the issue occur only with Chinese filenames for instance, or also with pure-ASCII names? [IE _should_ be sending URLs as UTF-8 by default so that shouldn't make a difference, but it's good to check these things.]

GregUbben wrote:

The filename doesn't matter; it happens with every filename. I just retested with a simple (24K) uploaded Word document titled "Test.doc". It will start a download progress popup, echoing back that it is downloading "Test.doc" and showing 0% progress, but it is immediately overlayed by two more error popups - one from IE saying it couldn't find/access the resource, and one saying that WINWORD.EXE is dying and a log is being generated (don't know where this goes).

I assumed there was a known problem with accessing some resources from IE address bar, but couldn't find much in Microsoft's Knowledge Base. Could be related to http://support.microsoft.com/kb/953166, another problem where a page works when clicked as a link but not when typed in the address bar.
Or saw another report that similar things happen if expiry HTTP header is set to now or in the past.

This reportedly also affects people who wish to upload a document then email its URL to a bunch of others. The links apparently don't work when clicked in Microsoft Outlook either. Work-around for now is to mail the Image: URL instead, requiring two clicks by receiving users.

GregUbben wrote:

I finally tracked this down a bit. It is due to the "Vary: Cookie" header in img_auth.php. Due to limitations in its caching engine, IE usually won't cache pages if there is a Vary header. (See http://blogs.msdn.com/ieinternals/archive/2009/06/17/Vary-Header-Prevents-Caching-in-IE.aspx)

So a work-around is to remove the "Vary: Cookie" header. Adding an ETag header is also supposed to help.

Note that this issue also has a large impact on performance on corporate intranets where Explorer is used a lot, since all images on the page are reloaded every time the page is accessed.