BleepingComputer.com: Prevent IE7 from sending an OPTIONS method request?

I've got a secured web site (login required), but whenever someone tried to download an Office 2007 (Word or Excel) file while using IE7, it makes them login again. If they cancel the login, the file is still loaded, but they'll be asked for their login each time they do it. If they do provide their login, it won't bug them again unless they close IE7 and then the next time they get a DOC or XLS, it does it again.

It doesn't do it in Firefox.

I checked the IIS log, and apparently(?) the problem is the OPTIONS method being sent from the browser. It shows a status code of 401, which makes sense since I was aborting the request. I've looked Office, IIS and IE7 up one side and down the other and I can't find any setting that could possibly disable the OPTIONS method. Do you know a way?

The log line is:
2009-10-28 15:43:14 (blah) x.x.x.x OPTIONS (/dir/) - 80 - y.y.y.y Microsoft+Office+Protocol+Discovery z.z.z.z 401 2 2148074254 1843 341 0

I don't know if it's relevant, but it's immediately followed by:
2009-10-28 15:43:17 (blah) x.x.x.x HEAD (/dir/file) - 80 - y.y.y.y Microsoft+Office+Protocol+Discovery z.z.z.z 401 2 2148074254 1843 341 0