XP Service pack 2 failure to update

Failure when Installation Status:
Initialization installization…done!
Installing Windows XP Service Pack 2(update 1 of 1)…

With Green bar 1/2 way across, I get the message “Access is denied” of prssed OK and the recieved the message “service pack 2 did not complete. Select OK to undo the changes that have been made.