When you use the Active Directory Domains and Trusts snap-in to establish a trust between two domains, you receive:

Invalid Domain Name. The operation is disallowed.

One possible reason for this error would be if the two domains use different codepages. The characters used in the domain name of a domain domain using the English codepage (437) may not be recognized by a domain using the French Canadian codepage (863).

The only way to guarantee the successful establishment of a trust is to have both domains use the same codepage.

NOTE: See http://www.microsoft.com/globaldev/reference/loclanghome.asp.