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So, here's another quick tutorial, this time about 'combo' viruses.  
What's a combo virus? It's a virus which can infect both COM and EXE files. 
Sounds good, doesn't it? Well, it's not complicated and you can figure it out, 
I'm sure. Here are just some guidelines and solutions to some problems which 
may arise. Oh, this is still non-TSR oriented, OK? When you're through with 
this, it's time to switch to TSR viruses :) First problem with non-TSR combo 
viruses is how to determine whether to search for a COM file or an EXE file. 
One way could be to use system clock - if it's an odd number, search for COM, 
otherwise search for EXE. The other way is to search for EXE first, and if there 
are none found, search for COM. Lately, EXE files are much more common than 
COM files. So this virus would primarily travel with EXE files, but when there 
are none, COM would be fine too.  Nice... 
I'm sure you won't have any problems checking if an .EXE file is really 
an EXE file... 
What about restoring the header or first three bytes? This could be a 
problem, since we don't know if it's a COM or an EXE. Or do we?
There's a simple trick. You just use a kind of Boolean variable. How? 
Look:
                           
              Virus_Start:             
                           .
                           .
                           .
              Infect_COM:  .
                           .
                           .
                           WhatAmI=1
                           .
              Infect_EXE:  .
                           .
                           .
                           WhatAmI=0
                           .
                           .
                           .
              WhatAmI db 1   ; 1=COM
                             ; 0=EXE
              Virus_End:

So when you need to restore the file, just check the WhatAmI variable.

                           WhatAmI=1?
                           jne Restore_EXE
               Restore_COM:.
                           .
                           .
               Restore_EXE:.
                           .
                           .

Simple, but effective! From now on, all infected files with your virus 
will
carry a kind of ID, saying: a)I'm a COM file
                                  or
                            b)I'm an EXE file

This is all there is to it. I don't see a reason for not including both 
COM and EXE infection in your new virus, do you?
Until next tutorial... Keep learning and have fun!

Greetings:
-First a BIG greeting to all SLAM members! We're in the house!
-Greetings to all virii writers in the world! (Keep up the good work!)