Stephane,
Do you have a C drive? When 6.1 installs it most have a C drive. If you do not have a C drive an error is generated. What you need to do to fix it is do have a C drive, the easiest way is to change your floppy to be drive C, and then place a floppy disk in the drive.
After the install is complete you can then change the floppy back to A.
Here is a link that discusses the error in a slightly different instance, but has the directions for the fix.