It seems there are as many ways to set up a windows network and proxy server as there are engineers setting them up. In my current place of employment I have found a number of apps including DropBox simply do not work across the proxy server, even with all the relevant details and passwords.
I have, however, found a solution in CNTLM (cntlm.sourceforge.net). This little utility can be run or installed as a service as desired and will provide support to any apps that have not implemented NTLM or have done it badly.
You install the application, setup your proxy server and account details and point applications to it instead of your corporate proxy server. Hey presto, DropBox springs into life.
Please note that while Cntlm can support http tunnelling this may annoy your sys-admins, a lot. I personally use it to provide SOCKS and NTLM support to apps that otherwise won’t talk to the internet over our proxy.
It is working a treat.