Accessing User Accounts in Control Panel

Hi all,
I’m running a fully patched WinXP machine and recently when I tried to switch on fast user switching I got the error: “windows cannot open the User Accounts control panel”. I cannot resolve the issue. I do not use system restore so that option is not available to me and I am reluctant to do a repair. any advice would be gratefully received. LKG does not resolve the issue, either.
Kind regards,
Excalibur909