Infected

Easy combos:
Get the chainsaw, and the combos will become easier. Stand over a Manhole, let
the chainsaw rip, and watch your combos, go over the top. Ones with lots of
enemies around them are the best.
You can get easily up to 10 combo with one slice.

Unlock Santa Claus and Bloodrayne:
Finish - all missions.

Getting avatars:
Kill the zombies near the slipknot sign in the following missions:

Slipknot 0 - Sign location at Time Square in mission 5.
Slipknot 1 - Sign location in the subway in mission 7.
Slipknot 2 - Sign location by the helicopter - in mission 10.
Slipknot 3 - Sign location in the upper left section of the map in mission 13,
Slipknot 4 - Sign location near the landing zone In mission 16.
Slipknot 5 - Sign location in Time Square in mission 20.
Slipknot 6 - Sign location in the bottom section of the map in mission 23.
Slipknot 7 - Sign can be found when the missionstart screen in mission 26.
Slipknot 8 - Sign location in the bottom right section of the map in mission 31
Chimaira - Sign location in the long street section under the park in mission 29

Getting the 1 avatar:
Type the cheatcode when examining the 1 Avatar:
Use the R to spin. Spin twice on the 2nd spin stop, after press:
Triangle X,X then Triangle Circle