My pc says windows could not start because the following file is missing or corrupt: \WINDOWS\SYSTEM32\CONFIG\SYSTEM.

The file that is missing / corrupt is one of your System Registry files.

By far, the easiest way to recover from this is to go to another computer and create a free Hirens Boot CD.
   Hirens Download: < http://www.hirensbootcd.org/download/ >  (Link at bottom by disk icon)
   Burning Hirens CD < http://www.hirensbootcd.org/burning/ >

Once you create the CD, then boot your problem system with this CD.  At first window, select "Mini Windows XP" and wait for the system to boot.  When it boots, go to the Hirens Boot CD menu and select 
    Registry ->  Registry Restore Wizard.
then follow the prompts.  This wizard will restore a copy of your Registry from the backups that are kept with your System Restore points.  After restoring, shutdown, remove CD, and reboot.

When your system boots, it is recommended to perform a System Restore operation to assure coherency in your system
   " How to restore Windows XP to a previous State"
   < http://support.microsoft.com/kb/306084 >

Also, this error is almost always accompanied by, if not caused by, File System corruption.  It as also a good idea to perform a Disk Check (chkdsk) operation with the "Repair" option
   " How to perform disk error checking in Windows XP"
   < http://support.microsoft.com/kb/315265 >

The hard way to recover is covered in the following Microsoft KB article but if you've installed a lot of service packs to your XP installation, I don't give you much a chance of getting past Part 1. (And it also requires an install CD to get to Recovery Console)

" How to recover from a corrupted registry that prevents Windows XP from Starting"
  < http://support.microsoft.com/kb/307545 >

HTH,
   JW