I have never had this problem while play, Minecraft on my windows vista.

Problem: 

-No hostile mobs seem to spawn

Deletes:

- playing  minecraft 1.7.2
- using the mod Rei's mini map mod
- tried already to check if "mode" is on peaceful... BUT NOT, on normal
- on windows vista
- problem only started a week ago


**No Zombies, No Skeletons, No Adventurer in Caving! No Point in "Nighttime", No point in playing**                 :O