PC cannot boot up

Wow.
My first suggestion is short the reset BIOS jumper and power on the computer. Allow BIOS to POST. If thats not possible, pull the CMOS battery off of the MB for a few minutes (15 is the most you should need, probably no where near that) with the power cable unplugged.
I would also try unplugging all peripherals except the video card, including all IDE devices, mouse, keyboard, USB, floppy. If it still doesn't boot, try a different video card.
If you are able to get BIOS to POST after this, start adding hardware back into the system, one peripheral at a time. If it stops working, you know where the problem is.
If instead, you are unable to get it to work after trying new video card... sorry.... It's one of three things.
1. CPU
2. Motherboard
3. Memory
And for my sanity, I better ask this... you tried a different Monitor... right?

Well the Virus Scan couldn't have allowed this to happen you have to have a Hardware Problem here as well.

I would recommend resetting the BIOS by either changing a Jumper or removing the Battery. If you are going to remove the battery remember to first remove the Mains Lead or you run the risk of doing damage if you drop the battery onto a powered M'Board. A couple of minutes should be enough unless you have a IBM Think Pad T40 or similar as that will never allow you to flush the BIOS.

After that restart the computer per normal and you should get an error message saying that there is a Check Sum Error in the CMOS or BIOS and press F10, Delete or whatever to continue. When you press the correct key all you should need to do is reset the Date & Time and then try booting up and see what happens. If you don't even get a POST Screen to appear make sure that the mains lead is correctly plugged in and try again.

If that doesn't help you'll have to take some more drastic action and depending on your experience it may need to be sent to a shop for repair or if it's still Under Guarantee make arraignments to Return it for repairs.

Just make sure that when you replace the Battery the markings are away from the M'Board so you can see them when the battery is fitted.

Col

U SHOULD USE AN BIOS UPDATE.
HOPE THIS WILL WORK FOR U
BYE

Well in that case you can only look at the Power Supply, M'Board or CPU.

You'll need to inspect things and pay particular attention to the Electrolytic Capacitors near the CPU these look like tin cans standing on their ends. If any have bulged or are leaking the M'Board needs replacing. If they are intact then the most likely culprit is the Power Supply particularly if it is an unbranded one and finally the CPU.

If you have overclocked the CPU they will die soon rather than latter or if the heat sink has become detached they will die in seconds if that long. Also if the Heat sink Fan is no longer working or there is a buildup of muck in the veins of the Heat Sink it's quite likely that the CPU is now dead more so with AMD's but it can also happen with Intel's.

Provided that before this happened the case had remained closed it's unlikely to be a physically problem with the unit though I've seen some poorly assembled units where tie wire has been used and this has shorted out on the M'Board killing it.

You could also try reseating any plug in cards and push on the CMOS chip just to make sure that it hasn't part way worked its way out and is no longer making correct contact.

Col