Permissions accessing a MSSQL database through ISAPI DLL on a different machine

Hello,

We have two NTservers on the same domain, a primary and a second one defined
as NT SERVER (not secondary).
The first (main server) has its own IUSR_MAINSERVER and our MSSQL 6.5
server.
The second one is our Web server (IIS4) with his own IUSR_WEBSERVER of
course.

When my ISAPI DLL placed on the Web Server  wants to acceed my SQL database
with user SA on the Database component (standard security access on MSSQL),
I get this message:

Exception: EDBEngineError
Message: General SQL error. [Microsoft][ODBC SQL Server
Driver][dbnmpntw]ConnectionOpen (CreateFile()).
Alias: ADRESSES

I have declared the IUSR_INTERNET (with full access) in my MSSQL Server.
One precision, the same DLL works perfectly if IIS and MSSQL are on the same
machine.

Is it impossible to do that?

Thanx