Windows 7 - Secured website access with IE 8.x...

I have an IIS 6.0 server on W2K3 that provides access to a lot of documentation, both MS Word and Excel. The site is accessible over the Internet via SSL and requires authentication. There's no issues with logging in to the site, browse around, and accessing word documents.

The problem start when the excel spreadsheets are accessed, every excel spreadsheet requires re-authentication. Clicking on the link to the spreadsheet the authentication window pops up. The authentication credentials can be entered and the document is opened in excel. Alternatively, twice canceling the authentication window allows the document to be opened.

The account permission is the same for both the word and excel documents and yet, it's only excel that needs re-authentication. The same website works just fine with Firefox, Opera, and probably other non-IE browsers as well. With anonymous access set on the server, there's no issues with opening excel spreadsheet; however, this is not an option due to the content.

It seems that IE 8.x does not cache the user credentials and/or does not pass them over to excel like other browsers do. I've tried to add the website to the "Trusted Zone", but it had no effect on IE.
Does anyone have a suggestion for overcome this issue?
TIA...

Cr00zng