The PC configuration is: Pentium4 Intel,DDR2 256mb Ram 533MHZ(But now I am upgrading it to 1GB)
Whenever I tried to install Ubuntu, it says "KERNEL ERROR, Try with noapic option"
So what should I do now?
Will an increase in RAM help me?
Lack of Memory (RAM)
256MB is a very low specification for Ubuntu.
Ubuntu will work with this low specification - but only if you use a lower requirement ubuntu spin such as Lubuntu v11.10.
If you have more memory available then my recommendation would be Xubuntu (v11.04 or v11.10).
If you intend to stick with 10.04 Ubuntu then probably 512MB (preferably more) would be required.
Kernel error message
As to your kernel error - some older PCs have buggy BIOS and ACPI (power management) control.
What the error message is suggesting that you boot with the kernel parameter
I've grabbed a screenshot from another question I recently answered - just substitute the word persistent as shown with the word noapic
To display this screen, immediately when you see the purple screen with the symbol at the bottom (seen immediately after the boot sequence starts) press Esc... If you got to the "try ubuntu screen" you missed the opportunity!
When you press F6 a pop-up menu will display. Press Esc to make this menu disappear then use the keyboard arrow keys to move to the position shown in the picture.
Once you have installed Ubuntu - you will then need to temporarily boot with noapic - this is described in the linked question below.
When you have booted into your Ubuntu desktop you can fix noapic to be a permanent Grub entry - again this is described how to do it in the linked question.
Same error here it seems to be a bug in kernel 3.0 . Just goto your bios and disable hyper threading and you should be fine. I'm sure there working on this bug, I hope they fix it soon.