asp.net intranet and NT inaccessability

[gadgetfbi]

I have written a small time tracker app for my current employer for internal use, to save on logging in etc I have used the visitors windows account to authenticate the user and turned off anonymous access in IIS - problem: everyone in the office can view and use the site fine except one person sitting at a WinNT box. Same ie version, valid network account etc, just cant work it out. Does NT pass windows authentication details in a different manner or sommit? Any ideas would be greatly appreciated. tia, rob