I have the exact same question as another user posted some time ago.
Suddenly I can't download xls or doc files from outside the internet. pdf or zip and other formats works fine and surfing to the site works nice.
From inside our organization and from within the same subnet it works to download xls and doc. No clues in the firewall, all seems to be working as it should.
I have IIS6 on a windows 2003 r2 x64 server.