I had that game too,i got it for my birthday when i was about 11 or 12 years old,download it here to play direct on your PC (no need for emulators,just install it) http://www.games4win.com/games/alien-3/
Cheers Tricky, will have a bash at that one later.
This oldie from the ZX Spectrum is still king of the Alien games in my eyes.