here is a simple pointer-scan table for Serious Sam HD on Steam which I tested on three computers. Included are pointers for health, armor and all kinds of ammunition. Enjoy!
It won't work for the secret level by the way, because the game does handle this one in another fashion... I'm assuming it might save the bonus level values to the executable (now which damn program does stuff like that?) since it changed one time while I was playing.
How to use this cheat table?
Install Cheat Engine
Double-click the .CT file in order to open it.
Click the PC icon in Cheat Engine in order to select the game process.
Keep the list.
Activate the trainer options by checking boxes or setting values from 0 to 1
I think Croteam come onto sites like this to make sure no one is cheating in their beloved games. Gunfire Goons tries to do the same thing, but they fail miserably.