First go to the install directory and open "config.txt" in Notepad. Scroll down to where it says "cheats" and change the 0 to 1, then save the file and close it. Press the relevent key in-game during play.

Code	Effect
1	Stop enemies moving.
2	Scroll through weapons.
3	Create an explosion.
4	Spawn every pickup.
5	Level up.
6	Add weapon pickups to the battlefield.
7	Bleed.
8	Make enemies bleed a little.
9	Make enemies bleed a lot.
G	Enable Invincibility.
Z	Slow down time.