This aint to bad really.....dont focus solely on boot times only though.

Has the systems multitasking capabilities improved somewhat?

How long do individual programs take to load when you have...

Not only should you check whats loading at startup but also what's running in the background.....Make sure your boot sequence is set to 'normal' and run through these computer maintenance steps:
...

yeah.....quite an expensive exercise for no result!

P.S Guess it can only be soft related or bad sectors on the HDD

Test and repair them with freeware apps from here:...

CPU's either work or they dont....unless its heat related, its unlikely that they fail causing random shuts then allow start up later on.

I was going to suggest its possibly software related with...

mmm....this is very unlikely, but you probably mean PSU which is also out of the question.

The monitor most probably comes on through its seperate AC power connector but you most likely see a 'no...

What exactly happens when you say it wont power up? Do you hear any fans or hard drives spinning up, any beep codes but no monitor signal or just plain old nothing?
No lights, no sounds, nothing?

Look this might be a stab in the dark, but have you tried changing the Hardware Acceleration setting in the Display tool in Control Panel from Full to none, when set at full, this can sometimes...