Start the first level, admire the cool spaceship for few seconds and open the two doors, go up near the Computer and enter the Intex Terminal pressing ALT key.
Loaded the main green page just type one of these codes and return into the game, the new level should load immediately.
You should also receive bonus credits to improve your equipment a bit.

This procedure will work into every level and every terminal. I have tested all of them twice, have fun!


Level 2  - AAJIGDDC
Level 3  - CGGHDGGDG
Level 4  - HDICICCII
Level 5  - IDHEHDGCC
Level 6  - IJIIDIHEC
Level 7  - CFDFEFEFJ
Level 8  - JIIJIIIIC
Level 9  - AAAABAAAA
Level 10 - CCGDGBBBB
Level 11 - HHIAAJJIG
Level 12 - GGDDJJHFD
Level 13 - JIECBFGFF
Level 14 - HGGEDDCCB
Level 15 - HHHGFGDCC
Level 16 - IHDCHGHFF
Level 17 - IHDCHGHFF (Restart from Level 16)
Level 18 - IHDCHGHFF (Restart from Level 16)

The last three levels are considered the final mission, if you aren't an expert I suggest you to restart from previous levels 
or your equipment may not be adequate, beware, you won't find Intex Terminals, no shopping for you =). 

Dave