Before starting on a full-scale examination of the computer, ask the user what he or she was doing before it crashed. Ask them if they altered any of the settings, downloaded any files/programs or installed any software. If they did, then that is the place to start.
If Windows does not start normally, you should try to start it in Safe mode, which bypasses the real-mode drivers (drivers that use the first 640KB of memory) and configuration and loads a minimal protected-mode configuration that disables the Windows drivers and provides only a VGA display.
There may be a problem with the registry. It may even be as simple as someone deleting the files from the root directory including the system files that should be hidden - it can happen.
If a PC boots to Safe mode but the problem remains unsolved after scanning the Registry (using SCANREG) and running the start-up process, the problem may be hardware related. The registry is where Windows records most of the settings for the computer including the hardware attached and the software installed. To check if there are any conflicts open Device Manager. Every device needs to be recognised by the operating system and is assigned an IRQ. With the number of devices that can now be added to a computer most plug and play devices will happily share an IRQ (Interrupt Request) number but if your computer has non-plug and play devices, these usually need a specific IRQ to function properly.
If there are no conflicts, isolate all the devices and reboot the computer in normal mode. If this works then enable the devices one at a time until the system halts. The last device you enabled is most probably the one causing the problem. To check, disable this device and enable all the other devices. If the system boots, then you have isolated the problem.
However, if Windows will not start in Safe mode, check that the CMOS of the computer has not been altered and scan the computer for viruses by booting from a floppy or other removable media and CD-ROM and then running a virus scanning program. Remember to protect your floppy disk and other removable media.