I don't understand. I did a third try and ms-dos booted (I mean from the hard). And I didn't touch anything. I had even turned off the machine with no results. And now ms-dos happens to boot!

But this is worse than before. I thought something was bad in the MBR. And and it was only a matter of fixing it. But now it may be the hard disk is having I/O errors. In any case it seems to be an intermitent failure, which are very hard to detect. Well, it's not so bad, because still the data remain accessible. The only issue is I cannot boot in ms-dos and, therefore, I cannot run some programs that only run under ms-dos, specially a chess program named chess.exe (the only one in fact). And also, of the four o.s., ms-dos 5.00 is by far the fastest. Regards.