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