You operating system is corrupted and needs to either repair or reinstall your operating system.
For repairing the operating system, you need operating system installer CD and when reparing, all the your software installed, document files and hardware drivers will still be there. But if your computer crashed because of virus attacked. It is not advisable to run the repair mode. Here is the link on how to repair the operating system http://www.fixya.com/support/t7687003-computer_suddenly_crashes_fails_boot
For reinstalling the operating system, all the software will be wipe out including your hardware drivers and document files will be losted. So be sure you have all the hardware drivers of your computer like chipset driver, LAN driver, VGA driver, Sound driver and others. Here is the link of how to reinstall your Windows XP operating system http://www.fixya.com/support/r7410371-reinstall_windows_xp_or_installing_clean
. For other like vista and windows 7, it is somehow the same procedure.
I hope it helps you and thank you for using FixYa !