What web server are you using? Apache
Tomcat? If so, check out JCIFS and their NtlmHttpFilter. I've used this at work myself; here is part of the web.xml file:
This works with all major browsers (Internet Explorer 6-8, Firefox, Chrome, Safari, Opera) in both Windows XP and Windows 7, although the security settings of the latter may need to be tweaked a little to get it to work with Internet Explorer 8. Internet Explorer won't even ask for a login.