Interesting:

Some corporate networks are behind HTTP proxy servers that use NTLM authentication. Dropbox currently doesn't support this kind of proxy authentication. However, some users have reported some success using an intermediate proxy, such as [WayBack] NTLMAPS or [WayBackcntlm, to get Dropbox running on their network.

This article explains steps on various systems to install proxies that support NTLM authentication: [WayBackAllow Dropbox to Authenticate With a NTLM Proxy Server - The Unofficial Dropbox Wiki.

Need to try these. Maybe they work for Copy.com too (:

Some other links around HTTP Fiddler that might be relevant:     Read more of this post