It tells the browser it is a download, so that most downloads skip the display and go directly to save-to-disk and ask for a filename. Does it here under Linux firefox on FreeBSD.
However, when I telnet I get
Connected to download.nvidia.com.
Escape character is '^]'.
GET /freebsd/1.0-8174/README/index.html HTTP/1.0
HTTP/1.1 200 OK
Date: Mon, 12 Dec 2005 11:52:32 GMT
Last-Modified: Wed, 23 Nov 2005 12:19:52 GMT
Which doesn't have the HTTP header so say the above (forgot what it's called).
So I think this must somehow react to the client software.
Clock is off by several minutes, too, BTW.