^^^ Well for starters, if your are comfortable with dissablembling your cpu and the heat sink/fan that sits on it, then take out the cpu, clean off the fan for the dustand crud , clean off the older thermal paste from both cpu and heatsink, apply a small amount of fresh thermal paste and reseat the cpu alongwith the heatsink.
If you are not comfy with whatever i have mentioned above take it to your local dealer and ask him/or a firend to do the same. They thermal paste that the shopkeepers wil luse is usually a 10-20 rupee satchet. Thats the input cost. What they charge for "servicing: ur pc will depend on their pricing policy and how much you can negotiate.Under no circumstance pay more than 100 bucks !
Incase the cpu hetsink fan is gone, assuming yours is a socket 478 mobo, a replacement heatsink + fan from zebronics will come at around 180 bucks, thats about the best cooler you can get readily in the local shops.
Secondly please clean off the interiors of your pc since now you have this wonderful reason to do so
And try to make the interior free of cable chaos
...
Cheers.