Didnt know that there is such a problem. I just tested it myself on my dual core and had no problems to host and join multiplayer lan games, from inside the game and with the dedicated sacred server.
But even if you are right with the multi core stuff, I doubt that you really have to do the stuff with the boot.ini. Just force single core usage! I know that you wrote, that this doesnt help and I know that a guy in the forum said it doesnt help. I just think you / the guy did it the wrong way, by using the task manager
AFTER starting the game, right? This often doesnt work with games that have problems with multi core systems.
The solution is to force single core usage
BEFORE starting the game. For this task there exists a command line command which is called "start". To get informations about how it works, you just have to type "start /?" in the command line, which can be reached by pressing windowskey+R and typing in "cmd".
The easiest way is to create a file in the same directory, called start.bat (it have to end with .bat not .txt or anything else, keep in mind that windows often hides the real extension, by the way one of the most stupid things Microsoft could ever do, and they did many of them...). After you have created this file (with the right extension), you could write in this for example:
start /affinity 0x1 sacred.exe
After this, you just have to doubleclick this file to run sacred on one core from the very beginning.
Another way to run sacred on a single core while on a multicore system without doing such stuff like you mentioned would be to run the game from inside a virtual machine that has one core.
Edit: I found out, that I already run sacred.exe forced to one core, by using the Radeon Profiles Tool. I did it a long time ago, because I often run other software while playing sacred and sacred took sometimes to much CPU. That is why it was working for me, because this tool is also able to force single core usage BEFORE starting the game, like the described way above.