Abhishek1025 :
When should i change the thermal paste , or when should i consider changing the thermal paste ! what should b the time interval in changing the thermal paste? my pc is 5 months old!
Changing the thermal interface is totally dependent on whether you are the one that applied it in the first place. All heat sinks that come with pre-applied thermal compound material that melts to seat, is usually always too much compound needed for the best heat conductivity. In most cases you can get better heat transfer just removing the pre-applied and properly applying new thermal compound.
The illusion that the thermal compound will last indefinitely is not true, it will dry out over time some much worse than others do. I do so much testing that my thermal compound applications usually never see 3 months, so I do get to see how each thermal compound tends to hold up.
If you're a set it and forget it type of computer user I'd say change the thermal compound at least once a year, that way you'll get to see how the compound is holding up, if it is seriously dried out lower that change out time interval, there's nothing better than a visual inspection of your contact footprint.
It's always a good idea to pull your heat sink and inspect the thermal footprint so you know exactly what your contact is too much thermal compound insulates instead of conducts the heat, you only need enough thermal compound to fill the imperfections between the two contacting surfaces.
I've seen long term situations that the thermal compound was so baked on the heat sink did not want to turn loose of the CPU and actually pulled the CPU from the socket and those machines were never overclocked. You are asking this question in the overclocking section and overclocking adds more heat, so it's just smart to keep an eye on the thermal situation, especially if you are overclocking.