Trying to install NetBSD was a disaster .....

That's really not a good way to start asking for help.

   The result : both my drives were "killed in action" (Parttion table distroyed).
   I managed to save one of them : my DOS - partition (thank got!), but I lost 
   my second DOS and both my LINUX partition (that's OK, you say).

To quote the installation notes:

`First and foremost, before beginning the installation process, MAKE
SURE YOU HAVE A RELIABLE BACKUP of any data on your hard disk that you
wish to keep.'

   labeling disk wd1 .... wd1:cannot find label (no disk label)
   overwrite disk with DOS partition table (y/n) [n] ?

It will only ask that if you screwed up either the size or offset of
the `NetBSD portion of the disk' that you told the script.

   * If I answer YES than BSD can mount the drive and labels it, BUT I 
   overwrite my partitontable and makes a partition of 25 M, and destroys the 
   rest!!

That really is what you told it to do.  But since people don't seem to
get the idea, I've just changed the message to `Erase the previous
contents of the disk? [n]:', which is really a lie, but perhaps it
will get the point across.

   Another thing is that I need a boot selector.... Do you know a good one?

OS-BS works for me.