1.How far along in boot process does the system get(POST - BIOS Screen - Windows Loading - Windows Login Screen - or Desktop)?
2. Does "F8" respond to get to Advanced Startup Options before restart?
3. Before this started did you have any signs of a malware infection on this system?
4. Have you recently installed any new hardware (anything from internal devices to external devices)?
You have to create a new boot file for the system:
1 Boot with Windows XP cd
2 Once in the Microsoft Setup menu press R to open the recovery console.
3 Select the operating system you wish to use; if you only have Windows XP on the computer you will only have one prompt.
4 Once prompted for the password enter the Admin password and press enter.
5 Once at the command prompt type bootcfg /rebuild to start the rebuild process.
6 The rebuild process will step you through a number of steps depending upon how many operating systems you have on the computer and how the computer is setup. Below is a listing of the common steps you are likely going to encounter.
Though the previous post may have a 65% chance of working, you should warn the user that this could cause further issues if not the problem. Recovery Console, if used incorrectly, can lead to further complications. A corrupt MBR can also lead to a similar issue - Symptoms: Blank screen after POST, Error: Missing Operating System, Error: Error Loading Operating System (malware might be the cause). The user has not given enough info on how far the boot process gets before restarting. They could try your fix but with due care.