We are in a process of upgrading to Windows 2003

We have a application running under MSDTC which is running fine under Windows 2000.

If we put the same application under MSDTC of Windows 2003 .While browsing from the browser we are getting below error

"New transaction cannot enlist in the specified transaction coordinator."

We have noticed that if we select the "Supported" under Activation Tab it works.
But under "Required" it doesn't work.

Can someone help please