The problem is solved, and I think other should like to know how.
I can't tell why I got the problem, but I did all I could according to the instructions. At the end the PC only booted Win Vista and AirBoot did not show up. I have two disks. One with Vista and one with eCS. According to the instruction AirBoot would be installed on the first disk. I never tried to tell the installation program where to install AB. I just got the info that the installation was succesfull. The first disk was the one that was installed when I bought the PC. Here Vista was installed. I later bought a new disk (SDD) and there I installed eCS 2.1.
Then a couple of weeks ago the problems came as described earlier.
The solution was to go into the BIOS and change the booting sequens: disks, CDs and so on. Now the SDD disk with eCS 2.1 is the first disk where the PC try to boot from. May be this is the disk where AB is? I guess so. Well, why did all this happend? An explanation I have got is that the internal battery(at least 5 years old) (for holding settings etc) in the PC shold be replaced. The setting was lost somehow and the PC would start from disk 1 where there was no bootmanager, but only Vista.
May be there is another explanation or that something else is wrong, but anyway I now can boot the OS I want from AB.