Installing with MSSQL

I have been using Mantis 1.0.8 successfully since February but have just

changed my webhosting package.  I now have a Windows webserver with 
MSSQL but cannot get Mantis to install.

When I try to run install.php I get this:

BAD
database is not supported by PHP. Check that it has been compiled into 
your server.

with an associated error
*Fatal error*: Call to undefined function mssql_get_last_message() in 
*\\........\mantis\core\adodb\drivers\adodb-mssql.inc.php* on line *492

*I am not sure what to do.  PHPInfo shows my webserver to have PHP V 5.1.4*
*
Can anyone suggest how to get this working?

Peter

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