First, get the rocket launcher. Then get the Long Jump Module. Next climb the two ladders near where you first found the long jump module. Then, next to the top ladder there is a rail. Jump onto it. Look strait forward. You can see the roof to the building. Use the long jump module to jump onto the roof and massacre your opponent with the rocket launcher. You have to time the jump perfectly to do it right,, and reach the roof.
There is two sides to the main building. Go to the side with the security desk (the side with no rocket launcher). Both players jump onto the big rock. Next, both players should jump onto the material which is above the entrance to the building. Jump on top of your opponent. You should now be able to jump onto the building. By the way, you need the 'Xen Gravity' cheat to be active for this to work
Press Left, Square, Up, Triangle, Right, Circle, Down, X at the cheat screen.
Press Down, X, Left, Circle, Down, X, Left, Circle at the cheat screen.
Slow motion mode:
Press Right, Square, Up, Triangle, Right, Square, Up, Triangle at the cheat screen.
Press Left, Square, Right, Circle, Left, Square, Right, Circle at the cheat screen.
Press Up, Triangle, Up, Triangle, Up, Triangle, Up, Triangle at the cheat screen.
Press Up, Triangle, Down, X, Up, Triangle, Down, X at the cheat screen.
Hint: Starting tips:
After the "accident", you will discover a scientist giving CPR to a security guard. Wait 15 to 20 seconds to see the outcome, which usually results in the security guard being revived. He is not useful at this time, but return to him after getting the crowbar. Kill him with the crowbar so he can not fire back to get his gun and ammunition.
Soon after this, you will ride an elevator upward. Have the gun ready. Shoot the monster attacking the security guard; a couple of shots should do it. This security guard can cover you until you reach the main entrance area.
One important tactic in the early game is to use your "allies" to your advantage. When you no longer require the services of a security guard, kill him and take his ammunition. A scientist is also useful in drawing enemy attack. Be careful not to harm an ally when another security guard is present, as he will attack you. Also be aware that monsters can sense your allies; and even if you crawl, monsters can hear your allies' walk.
The beginning of the game also has the following glitch. There is a scientist sitting in front of a damaged instrument panel, in the area of the first scientist who opened a door for you. This scientist is invincible. You can shoot or beat him with a crowbar, and all he will do is whine. This also saturates the area with a large amount of blood.
Hint: Saving ammunition:
Do not waste precious pistol ammo on headcrabs. Use your crowbar, as it is more useful.
Hint: Using the pistol:
Use the following trick to save ammo for all guns other than pistol. Most creatures or persons can be killed with five or less direct shots of the pistol.
Crabs: 2 shots or 1 crowbar hit
Zombies: 4 shots
Electric Aliens: 3 shots
Trip Bomb: 1 shot
Machinegun on stand: 5 shots
Soldiers: 5 shots
Dogs: 3 shots
Spitters: 4 shots
Hint: Using guards:
If you have a guard, get him close enough and he will take care of almost anything for you. The only time you ever need to fire is if the creature is close to the guard.
Hint: Barney gets cut in half:
When you enter the room where the lasers are shooting out the walls, run through. There should be a Barney (security officer) on the floor. Watch carefully and you can see the laser go across and literally cut him in half.
Glitch: Floating doctor in wheelchair:
Note: This trick requires a second player. When playing a Decay game with another player, get to the Rift level. When the handicapped doctor tells you to follow him and opens the door, do as he says. He will get in the elevator. Block his path so he cannot get in the elevator, but the door is open. Next, back up slightly and let him enter. Get behind him and run forwards while the other player pushes you from behind. The doctor should start to go down, and the door has closed on you, but it is still open. Jump frequently and press Forward quickly. If you jump onto the safety bar where the top of the other door is, he will get to the bottom of the elevator. If you stand on the bar, and he gets to the bottom, then he will launch back up and will float in the air. You cannot kill him with a regular gun from below. You must shoot him with the bazooka or from above to kill him.
Enter the codes in the cheat menu before you play the game.
left square up triangle right down x
down x left circle down x left circle
left square right circle left square right circle
right square up triangle right square up triangle
LOOK LIKE AN ALIEN
up triangle up triangle up triangle up triangle
up triangle down x up triangle down x
To get slow motion on half life for PS2, go to the credits in the options menu and go all the way to the last page, where there is a cheat u have to put in in the 'enter cheat' menu.