login required when accessing a virtual directory....

Hi,

I'm using Win 2003 and IIS6

I have a web site (which is a sharepoint one), and a virtual directory which
run under another application pool.
the content of the virtual folder is displayed in IFrames under my first web
site.

I have a domain with a lot of sub-domains, the active directory mode is Win
2000 mode, not 2003.

the security is set to clear/text for the 2 virutal directories.

when a user access the root folder, the login is asked and all works fine.

but, if the user is a member of a subdomain and if during the first login
the user only type his login WITHOUT the domain (toto instead-of
domain\toto) then the user is asked a second time when the iframe will
display the content of the virtual folder.

if the user type domain\username for his initial login, then the user is no
longer asked for a password again.

any idea?
what can I do to prevent this second login?

thanks.

Jerome.