Luis,
It depends. Do you want your program to control autocad before, during and after your startup command executes, or do you just want to start autocad and run a command?
If you just want to start and 'fire and forget' a command, you could do this:
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
namespace AcadStartupScript
{
class Program
{
static void Main(string[] args)
{
ProcessStartInfo acadStartParams = new ProcessStartInfo();
acadStartParams.FileName = @"C:\Program Files\AutoCAD 2010\acad.exe";
acadStartParams.Arguments = @"/b C:\Temp\LeStartupScriptOMatic.scr";
Process acadProc = Process.Start(acadStartParams);
}
}
}
...and the script file alluded to above is thus:
(alert "G'day Luis!")
If, however, you want control, then I would suggest the COM route, as you've already mentioned. I believe there are a few examples here and on the adesk ng's, but I've not done it.