Stop : c0000218 SAM error ???

i encounter problem with the follwing blue screen information on my w2k server sp2 :

STOP: c0000218 {Registry File Failure}
The registry cannot load the hive (file):
\SystemRoot\System32\Config\SAM
or its log or alternate.
It is corrupt, absent, or not writeable.

any one can help me to resolve this problem ??