Intel Pentium E5200 2.5GHZ Overclock

Solution
Microsoft Security Essentials isn't a stress test. Using Prime95 or Intel Burn Test will make it run hotter than 51°C. At 100°C it will throttle and the frequency will drop.
Monitor the temperature and frequency while stress testing it. You can increase the overclock until it becomes unstable or it starts throttling because it's too hot. Your CPU may not be able to reach 3.15 GHz, but it could possibly exceed it. We can't tell you the maximum that your CPU will achieve; you'll have to find it.
 
You should be fine up to around 1.4v vCore and 74°C Tcase, which is the temperature of the CPU package, not the cores. Those are usually 5-10° higher so aim at below 80-85°C on the cores. This is the temperature Intel publishes as a safe temperature for long term use.)
Depending who you ask, values of 65-80°C on the cores are the safe limit.
And even then there is the so called "TJmax",the temperature at which point it gets critical. Those are usually around 95-105°C, depending on the model and are the point where the chip starts thermal throtteling