>> Any life-form intelligent enough to come half way 
>>across the universe to annihilate us would surely have Norton Anti Virus 
>>on their computers.
>

NONONONONO

You simply didn't understand just how clever Jeff Goldblum was. 

Firstly, he realised that the aliens were probably using a
virus-checker that recognises viruses by their signatures. Such
virus-checkers are only effective against known viruses, so he knew
that if he wrote a new virus it would probably slip through the
aliens' defences.

Secondly, he recognised that all viruses are hardware-dependent (a
virus that infects a PC cannot harm a RISC machine, and so on). 

So....

a) He reverse-engineered the computing hardware in the captured alien
spacecraft and deduced what instruction set their computers supported

b) He wrote an alien-hardware emulator to run on his laptop, so that
he could use it as a development and test platform

c) He analysed existing comms software on the captured alien
spacecraft to determine what communications protocols they use, so
that he could upload the finished virus (after all, no self-respecting
alien would use TCP/IP)

d) He analysed the alien operating system, to determine what the
virus warhead would have to do in order to cripple it (a virus which,
when triggered, did nothing but write "please go away" to the aliens'
monitors would be of limited effectiveness, I imagine)

e) He wrote and tested the virus (in machine code, of course, there
wouldn't have been time to invent an assembler language for the alien
hardware and write an assembler for it)

f) And finally, he wrote a small comms program to allow his laptop to
"speak alien" to the computer on the alien mother ship.

Of course, he was lucky that the aliens used the same physical and
datalink protocols as we do (it would have been very difficult to
change those in the four hours available to him to complete the
project), but he was due some luck :-)

The rest was simple. Of course, he should probably have explained all
this, when he was expounding his plan to the military. But, as anyone
who has ever been involved in software development knows, there is no
point in trying to tell management about the problems you've had to
overcome; managers want to see a working demo, nothing else interests
them. So he was probably right not to try to explain.

So it wasn't far-fetched at all, just inadequately explained.

HTH

Tony Gardner