|You could very easily have dust build up on heat sink cooling fins, and it can get thick enough to block the cooling fan from turning, especially the CPU and Graphics fans. Use compressed air can for this purpose (Radio shack, etc.) and make sure all fans are running properly before closing case again.|
If you want to check your temperatures and fan RPM's without opening your case, download Speedfan (oveclocker's tool) that can give you direct readouts on these (CPU core temp, GPU core temp, chipset temp and some fan RPM's) within windows.
You have to be a little bit crazy to keep you from going insane.
If all else fails, read instructions.