I've confirmed this and SilverStripe works with IIRF on Windows Server 2003 R2 using IIS 6. I also used PHP 5.3.3 and MySQL 5.1.51.
Unit tests pass, even:
php sapphire\cli-script.php dev/tests/all
851 tests run: 851 passes, 0 fails, and 0 exceptions
Maximum memory usage: 80.2M
Total time: 124.351 seconds
124 seconds for a full pass isn't bad for an older copy of Windows! For performance, it's also important you update everything on Server 2003, including .NET Framework updates and service packs (important!). Then you can use a VC9 non-thread safe version of PHP and the latest MySQL 5.1.51. I installed my copy of PHP manually, as well.
Not only that, but the /admin URL is working correctly. The main thing is you need to create a new "vhost", which means creating a new entry under "Web Sites" in IIS 6, pointing it to a location in C:\Inetpub\wwwroot\silverstripe (or the directory you want) then putting an Iirf.ini file into that directory for rewriting to work.
I've got the start of a step-by-step tutorial, which also explains how to install FastCGI and PHP on Windows Server 2003, so I'll finish that up and post it on http://doc.silverstripe.com
EDIT: Guide available here: http://doc.silverstripe.org/installation-on-windows-server-2003-iis-6