Hi,
If i switch off power supply for few hours(say >6hrs), then pc does not start immediately after switching on.
It takes about 15mins or more (depending on power-off time) after which it starts.
I also tried changing cmos battery with a new one, but still no improvement.

Please help to detect & resolve issue.

'PC does not start' or 'PC takes a long time to start' is kind of dubious. What exactly happens? Where is this delay?
Is it the BIOS - the fist screen where you see the name of the vendor or perhaps the make of the motherboard?
Is it after the BIOS when the OS (bootloader) does not come up soon enough.
Or is it after the OS where the OS takes a long time to get to the shell or the desktop?
The answer to all these questions is different so it would be helpful if we knew where exactly we have this delay.
Hope this helps.

It is 'before' all stages you mentioned.
I think writing things stepwise might be more helpful to explain the problem.

1.PC(running)
2.PC(shutdown)
3.Power switch/Spike guard/Suppy cord switch - off(h/w button)
4.Waited for few hours
5.Power switch/Spike guard/Suppy cord switch - ON(h/w button)
6.Pressed CPU cabinet power on button
7.PC does not start(nothing shows up on monitor, whatsoever)(small led lights on cpu cabinet body glow when i keep that button pressed, but PC does not started)
8.If i keep the PC like this(power plug/spike guard/supply ON) for sometime(>15mins) and then again try to start PC by pressing CPU cabinet button, then PC starts as usual.

I've not much faith in long distance diagnosis, but here goes.

I'm a hardware guy. My guess is the power supply is shot. Specifically, there's small high value resistor(s) to provide a bleed current of a few volts to kickstart a power supply, after which it powers itself. These high value resistors (~ 470k 0.5W) are too small to be reliable - I always replaced with 1W ones, because the small ones die - why I don't know.

Give it a fortnight and it won't come on at all. Buy the PSU.

thanks for replying business_kid.
Good to have some idea about what might be going wrong.