Are you using Windows XP retail or an OEM version?
If your ..\config\system is really missing, its probably due to data corruption of viruses. You should try to run chkdsk from the Windows Recovery Console ([url]http://www.computerhope.com/issues/ch000627.htm[/url]) and issue the following commands:
Wait for it to scan/repair your file system and see what the results are. Then issue the command:
copy "C:\Windows\Repair\System" "C:\Windows\System32\config"
If your error message is a pop up box or something else other than an unpassable black screen with white letters, then you probably have some hoax virus. Lsass.exe has often been the causes of viruses, so it wouldn't surprise me if this is caused by one.
The above commands will restore a previous (and hopefully working) copy of your registry.
Check this page out: [url]http://xphelpandsupport.mvps.org/how_do_i_repair_a_stop.htm[/url]