Lsass.exe - system error HELP!!?

When booting my pc, I get the following message box "lsass.exe - system error" with the following explanation "Insufficient system resources exists to complete the API". The computer will only allow me to reboot over and over with the same message each time. Why do I get this message and how can I resolve this problem? Windows XP is pre-installed and I have no XP disks.