Problem in pc booting

[mic5]

Very often I observed that after switch ON my PC, it is not at all booting. Some time it boots without any problem.
In either condition, I am observing proper supply at the board supply points and no abnormal conditions.

How to rectify the problem? What may be the problem in this?

320GB hard disk with Ubundu & Windows Xp OS already been installed.

 

[alec_t]

Check that all internal and external connectors are fully connected and that all socketed components are fully seated.

 

[Ian Rogers]

You need to tell us how far the boot gets..

First the graphic card details show on screen (sometimes too fast to see)
Then the BIOS details..
Then the POST screen details..

Finally the operating system loads...

How far does it get?

 

[large_ghostman]

we had a really old pc that was used for a k190 data logga that plugged into the serial socket, in the end turned out was most the caps on the MB were leaking and bulging, as the machine was old it wasnt worth recapping, but it gave all kinds of strange intermittent faults on boot up until it would boot no longer

 

[Dr_Doggy]

have you tried hard reset, ie power cord, sometimes older pc s dont wake from sleepmode

 

[tunedwolf]

My first suspect would be the power supply, irrespective of the correct voltage being shown

As Ian already pointed out, we need to know if it's at least attempting to boot, when you say it's not booting, explain exactly what happens from switch on, step for step?