"I was really surprised, that Windows-NT still support direct access to the COM with inportb() and outportb() from a self written Borland-C DOS program (for flashing and debugging my 89C51 projects)."

It's the DOS virtual machine (or whatever they call it) that allows DOS programs to think they are accessing the ports directly. When you open a DOS box you're running a program that pretty much emulates a PC running DOS. This works fine under 2000 and XP as well.

The only thing you can't do (easily) is write a native WIN32 program that directly accesses port addresses.