IsWellKnownSid preventing Startup W2K Pro

Blue screened last Friday. Repaired, recovered, new drivers, etc. Finally got it to actually boot and look NORMAL at last. Everything was working. Windows started automatically updating. One security update wouldn't install (maybe number 14) because my system isn't new enough. Then it rebooted - but NOT! Got a little warning box about "IsWellKnownSid" and ADVAPI32.dll, followed by C:\WINNT\System 32|Services Status Code 128. Apparently it installed an update that requires IsWellKnownSid authentication before Windows 2000 Pro will open. From what I've been able to Google, you have to have XP to have that authentication. HELP!