Visual composer login problems

Hi,

I have installed VC6.0 on XP SP2, IIS 5.0 and MSSQL 2000.

Also I have NW 2004 with portal 6.4 on another machine.

To log iinto VC I type:

http://my-vc-server/vcserver

The application run OK, but, from VC I try to connect to my portal server defining this address:

http://portal-server:50000

Login windows shows up, but after input uername/pass nothing happens, the login screen keeps asking user/pass.

All InternetExplorer are V6, also have MSXML parser 4 and Adobe SVG.

Any idea will be appreciated

Regards