There are quite a few reason's the php_mssql.dll extension for PHP may not be loading on your windows server.
For PHP 5.2 anyway I found that both MSVCR71.DLL and ntwdblib.dll needed to be copied to various places.
MSVCR71.DLL needed to be in c:\Windows\System32 as having it in C:\WINDOWS\Microsoft.NET\Framework\v1.1.4322 wasn't good enough.
ntwdblib.dll needed to be both in the PHP directory and c:\Windows\System32
I'd love to know why this is, but for now I'm just happy it's working.