Boot problem whenever I turn on the computer, the HDD and power ON LEDs turn ON and stay ON and then turn OFF but the computer will not power ON. While this is happening, the screen is blank. please assist and advise...thanks

You were FORMATTING it? Meaning... you were erasing the operating system? If that's the case, then... you have no operating system and the computer cannot boot up because it has no operating system to boot into.

Im not sure what to do so If i where you i would bring it to a computer store and see what they can do.

You are the owner of a desktop PC that has developed a sudden and catastrophic hardware
fault - probably on the motherboard (since even the ROM BIOS doesn't give evidence of its
existence), but possibly in the power supply.

Desktop PC power supplies are (relatively) easily replaceable, so if you have as many spare
power supplies lying around as I do, try replacing that first - if that doesn't fix the system, then
the problem is on the motherboard.

Since your power supply is alive enough to light the power LEDs on the system unit tower and
also the keyboard, the problem is probably on the motherboard - look at it - there may be an obviously burned visible component - but there usually is no visible sign of the fatal failure.

The reason I do not suspect your HDD (hard disk drive) has crashed is that ROM BIOS
executes at (power start) boot time right out of motherboard-mounted ROM, independent of
whether or not the HDD is functional - and since not even the BIOS message appears on your monitor, I therefore know that the boot sequence has been interrupted long before the computer becomes intelligent enough to find and access a disk drive, and then command it to try to find a
boot sector.

How to fix the computer: isolate the boot problem to the motherboard. Replace motherboard.

If you have another computer that will hook up to the internet, download a copy of any live Linux CD (UBUNTU) and see if the computer will boot with that (you download a image file that burns to a boot able cd). If it will boot with a live CD, then you have a Hard drive / operating system problem. If it will not boot from that, you have a mother board problem. Start removing things from the motherboard, unplug and remove everything then plug hook up things one at a time.

Are you on battery mode?
If you are in battery mode, remove your laptop battery, plug in the adaptor to the laptop (without the battery) and plug to the power outlet. Turn on your laptop.

You can check the diagnostic LEDs with this information:
http://www.fixya.com/support/r4282412-find_no_post_diagnostic_information_dell
If you are getting 123 LEDs and a green/blue power LED the issue is most likely the hard disk drive, boot to the resource CD and run diags on the HDD

Check http://www.fixya.com/support/t3440299-presario_f500_laptop_problem

Remove the hdd cover from the bottom and make sure it is completely seated and connected. If the hdd is properly installed and it still will not boot, try entering the CMOS Setup. If you can enter setup you most likely have a bad hdd. If you can't you need to take or send the pc to a repair shop.

rest bios by removing battery and holding power button for 1 min.

Are you hearing any beeps when you start it up?