Drupal 6 on Windows with MS SQL Server

Please can anyone help me. I have spent about two weeks on this now and it's driving me crazy. I'm sure it's a common question, but I can't seem to find the complete (idiot-proof!) solution anywhere.

I have been using Drupal 6 for a while now - I have 20-30 separate sites all running happily on the same Windows 2008 server, IIS, PHP 5.3.3 and MySQL. The problem is that I want to use MS SQL for the back-end for a number of reasons. I have tried Drupal 7 and successfully installed with SQL server as the back-end. The problem is that the installation is a little flaky - I don't know if it's the use of SQL server, the fact that it's Drupal 7, or the Workbench Access module (which I had to use as a replacement for menu_node_edit) that is causing the problem, but something is and it's erroring a lot! Also, the range of modules for D7 is nowhere near as good as for D6.

So, I have downloaded php_dblib.dll and modified the php.ini file to include this. I downloaded a couple more files from a Microsoft development project that seems to have been working on this issue. These files were
database.inc
database.sqlsrv.inc
install.sqlsrv.inc
With these files I can get sqlsrv to appear in the database list at install, but when I put the correct connection info in, it says there is already a drupal installation in that database (there isn't, it's empty), and the installation cannot proceed. The account I am using has dbo access to the relevant database.

If I change the connection string of an existing site, I can get Drupal to talk to the database - it errors because the table don't exist, but at least it appears to talk to the server. However, I cannot get it to install to the sql database.

Please can anyone help? If I can get this working, I promise to write a step-by-step guide to help others!

Many thanks
Paddy