Port problem of Tomcat4.1 with IIS

I have installed Tomcat 4.1 with IIS . But Tomcat is not working when IIS  is active. It is working only when IIS is stopped .
But if I change port of IIS from 80 to 81 then both Tomcat and IIS working.

The problem is I do not want to change port of IIS from 80 to 81 . Since Proxy is also installed in my organisation. Changing   port of IIS means I have configure all proxy nodes on port 81 . I don't want this.

But similarly when I change port of Tomcat from 8080 to 8181  during installation and keep port of IIS to 80 , Tomcat   is not working . 

Any idea how I can change port of Tomcat so that IIS can still be on port 80 and both works fine.

Thanks in advance

S.K.Das