I Have been experiancing the same prob with oscommerce installed on my pc.
I downloaded the PHP4.4.2 add on - when i switch to php 4 and i try to access my catalog homepage I get the same error you get but when i switch to php5 then I can connect to the database.
"The flesh is willing, but the mind .... Hey who'se potato chips are these!!?"
you are using the wrong command to access mysql, you are using mssql_connect which is used to access microsoft sql server, not mysql. you need to use mysql_connect.