Is A.C. necessary for Desktop PC or Server, if i live in too Hot region? where summer temp. is 48 centigrade.
Definitely for a server you will need AC.
But if you have just one or two desktops in a room, I've found that they can handle it, although the hardware failure rates will be higher than normal.
I have done this with operator consoles at a factory I used to work at. And as long as it doesn't get above say, 50 degrees, and they are not in direct sunlight they will usually be ok.
Once you figure out things are heating up beyond normal,
Of course, if you have a lot of machines in the room, then it would make sense to cool the room (think of server rooms as against your work place with 2-3 machines in the room).
Update on your comment.
In short, I don't expect a great deal of performance increase by keeping your desktop cooler than say 30 C (I mean the temperature 2 inches above your processor, where the fan pulls in air towards it). But, I do expect potential performance and component life degradation if your system continues to run on higher temperatures.