Does ServletExec support Windows 2003 with IIS 6.0 ?
ServletExec 5.0 (both ISAPI and AS) has been tested with Windows 2003 (and 2003 sp1) and it works fine.
The New Atlanta website ran with SE 188.8.131.52 ISAPI on IIS 6 on Windows 2003sp1 for over 3 years beginning in 2004 and it worked just fine. In early 2008 that site was updated to run SE 6.0 AS on IIS 6 Windows 2003 R2.
Therefore this is officially supported.
If it does not work for you, then please go through each and every step in SE FAQ #7 (for both ISAPI & AS).
Note: In some cases, SE 4.2 may seem to work with IIS 6, however some have reported problems, and as indicated on the System Requirements Page, that combination (SE 4.2 with IIS 6) is not offically supported.