G3) I was infected by both Stoned and Michelangelo.  Why has my
    computer became unbootable?  And why, each time I run my favorite
    scanner, does it find one of the viruses and say that it is
    removed, but when I run it again, it says that the virus is still
    there?

These two viruses store the original Master Boot Record at one and the
same place on the hard disk.  They do not recognize each other, and
therefore a computer can become infected with both of them at the same
time.

The first of these viruses that infects the computer will overwrite
the Master Boot Record with its body and store the original MBR at a
certain place on the disk.  So far, this is normal for a boot-record
virus.  But if now the other virus infects the computer too, it will
replace the MBR (which now contains the virus that has come first)
with its own body, and store what it believes is the original MBR (but
in fact is the body of the first virus) AT THE SAME PLACE on the hard
disk, thus OVERWRITING the original MBR.  When this happens, the
contents of the original MBR are lost.  Therefore the disk becomes
non-bootable.

When a virus removal program inspects such a hard disk, it will see
the SECOND virus in the MBR and will try to remove it by overwriting
it with the contents of the sector where this virus normally stores
the original MBR.  However, now this sector contains the body of the
FIRST virus.  Therefore, the virus removal program will install the
first virus in trying to remove the second.  In all probability it
will not wipe out the sector where the (infected) MBR has been stored.

When the program is run again, it will find the FIRST virus in the
MBR.  By trying to remove it, the program will get the contents of the
sector where this virus normally stores the original MBR, and will
move it over the current (infected) MBR.  Unfortunately, this sector
still contains the body of the FIRST virus.  Therefore, the body of
this virus will be re-installed over the MBR ad infinitum.

There is no easy solution to this problem, since the contents of the
original MBR is lost.  The only solution for the anti-virus program is
to detect that there is a problem, and to overwrite the contents of
the MBR with a valid MBR program, which the anti-virus program will
have to carry with itself.  If your favorite anti-virus program is not
that smart, consider replacing it with a better one, or just boot from
a write-protected uninfected DOS 5.0 diskette, and execute the program
FDISK with the option /MBR.  This will re-create the executable code
in the MBR without modifying the partition table data.

In general, infection by multiple viruses of the same file or area is
possible and vital areas of the original may be lost.  This can make
it difficult or impossible for virus disinfection tools to be
effective, and replacement of the lost file/area will be necessary.
.