If you do get error messages like the one below then it means that your computer has an corrupt registry and you should do something about it.
Windows XP could not start because the following file is missing or corrupt: \WINDOWS\SYSTEM32\CONFIG\SYSTEM
The corrupt registry can be fixed manually, providing that windows restore is enabled. There is a lot of steps involved so here it goes:

1. Insert the Windows XP boot CD and boot off it.
2. Press ‘R’ to enter the recovery console
3. Select the Windows instance you wish to repair (remember to type the number and not just hit enter)
4. Make a temp directory by typing “md tmp”.
5. Create a backup of the current Windows registry by typing the following commands:
   • copy c:\windows\system32\config\sam c:\windows\tmp\sam.bak
   • copy c:\windows\system32\config\security c:\windows\tmp\security.bak
   • copy c:\windows\system32\config\system c:\windows\tmp\system.bak
   • copy c:\windows\system32\config\software c:\windows\tmp\software.bak
   • copy c:\windows\system32\config\default c:\windows\tmp\default.bak
6. Delete the current Windows registry by typing the following commands:
   • delete c:\windows\system32\config\sam
   • delete c:\windows\system32\config\security
   • delete c:\windows\system32\config\system
   • delete c:\windows\system32\config\software
   • delete c:\windows\system32\config\default
7. Copy the Windows repair registry by typing the following commands:
   • copy c:\windows\repair\sam c:\windows\system32\config\sam
   • copy c:\windows\repair\security c:\windows\system32\config\security
   • copy c:\windows\repair\system c:\windows\system32\config\system
   • copy c:\windows\repair\software c:\windows\system32\config\software
   • copy c:\windows\repair\default c:\windows\system32\config\default
8. The type ‘exit’ to reboot the computer.
9. Get into safe mode by pressing F8 before the Starting Windows screen comes up.
10. Log in as Administrator
11. Open My Computer and then the C drive
12. Click on Tools > Folder Options
13. Under the ‘View’ tab, select ‘Show hidden files and folders’ and uncheck ‘Hide protected operating system files’
14. Close the Folder Options window
15. Open the System Restore folder (if you cannot open the folder, please use this link to find out how to unlock it: http://support.microsoft.com/kb/309531/)
16. Open the ‘_restore{lotsofnumbers}’ folder that is not the current time (use detail view)
17. Inside the ‘_restore’ folder, select a recent ‘rp{number}’ folder, these are your restore points, use detail view for dates.
18. Inside the ‘rp{number}’ folder, open the ’snapshots’ folder
19. Copy the following files to c:\windows\tmp
    • _REGISTRY_USER_.DEFAULT
    • _REGISTRY_MACHINE_SECURITY
    • _REGISTRY_MACHINE_SOFTWARE
    • _REGISTRY_MACHINE_SYSTEM
    • _REGISTRY_MACHINE_SAM
20. Navigate to c:\windows\tmp and rename the files to the following:
    • DEFAULT
    • SECURITY
    • SOFTWARE
    • SYSTEM
    • SAM
21. Reboot the computer
22. Using the boot CD, get back into the Recovery Console as described above
23. Run the following commands to delete the repair registry:
    • del c:\windows\system32\config\sam
      
    • del c:\windows\system32\config\security
      
    • del c:\windows\system32\config\software
      
    • del c:\windows\system32\config\default
      
    • del c:\windows\system32\config\system
24. Run the following commands to restore the registry you copied from the restore point:
    
    • copy c:\windows\tmp\software c:\windows\system32\config\software
      
    • copy c:\windows\tmp\system c:\windows\system32\config\system
      
    • copy c:\windows\tmp\sam c:\windows\system32\config\sam
      
    • copy c:\windows\tmp\security c:\windows\system32\config\security
      
    • copy c:\windows\tmp\default c:\windows\system32\config\default
25. Type ‘exit’ to reboot the computer with the restored registry.

You have now manually restored your Windows XP registry!   This process can be done even quicker and automatically if you have a tool like a pc registry cleaner. But look if you can do it manually first.