I am using Winpkfilter 2.4.1. It works perfectly on all OSes except Windows 2003 Server. I tried changing registry entries for both Windows 2000/XP and Windows NT. IsDriverLoaded() function is returning FALSE.
There should be no problems specific to Windows 2003 Server… You should use the same registry settings as for Windows 2000/XP. I would recommend to remove everything (driver and registry entries), reboot and reinstall everything from the scracth.