How do I solve an 'lsass.exe' application error shown at the Windows XP boot?

"From your post, I understand that you cannot manage to boot or logon into Windows.
lsass.exe is a favorite target of viruses, and it's likely that a virus has destroyed lsass.exe while trying to infect your machine.

If you can boot into Safe mode (F8 during the boot), restore your system to an earlier system restore point. Then scan your disk using several anti-virus programs.

If you can't boot into Safe mode, try to boot into the Last Known Good Configuration (using F8).

If that also doesn't work, you should Perform a Repair Installation.

EDIT

You may also use a rescue live-CD virus scanner : I like best Avira AntiVir Rescue System because it gets updated several times a day and so the download CD is up-to-date."

I am afraid you will not be able to fix this one except with a reinstall. I would try the Repair installation that harrymc links to above, that's what we call an in-place upgrade. But I've had this issue before and once you mess with the Local Security Auth Server it is usually a lost cause.

"This is usually caused by a virus that is trying to load each time an application is initialized. The actual exe or dll may have been removed by a vrius scanner, but the registry key still remains. Theat is what is causing lsass to fail. There is a registry key that needs to be edited. but you can't load windows to get into the registry. you'll need a remote registry editor, like BartPE, or Ultimate Boot CD for Windows. THIS IS SERIOUS STUFF AND CAN F@#$ YOUR COMPUTER, so if you don't have COMPLETE CONFIDENCE that you know how to safely edit the registry, take it to a professional.

Once you gain access to the registry, back it up, then search for AppInit_Dlls. renamed it AppInit_Dlls.old. Then create a new string key named AppInit_Dlls. leave its value blank, empty. Save your registry changes and reboot into safe mode with F8. Once windows loads, install MalwareBytes scanner and do a fullscan. You will need to reboot to kill all viruses, and probably have to run the full scan a second time in normal boot mode (not safe mode).

Then install a serious anti-virus anti-malware scanner, nothing from McAfee or Norton Symantec will be good enough, they are just shills for suckers. thank you internet."
"This is usually caused by a virus that is trying to load each time an application is initialized. The actual exe or dll may have been removed by a vrius scanner, but the registry key still remains. Theat is what is causing lsass to fail. There is a registry key that needs to be edited. but you can't load windows to get into the registry. you'll need a remote registry editor, like BartPE, or Ultimate Boot CD for Windows. THIS IS SERIOUS STUFF AND CAN F@#$ YOUR COMPUTER, so if you don't have COMPLETE CONFIDENCE that you know how to safely edit the registry, take it to a professional.

Once you gain access to the registry, back it up, then search for AppInit_Dlls. renamed it AppInit_Dlls.old. Then create a new string key named AppInit_Dlls. leave its value blank, empty. Save your registry changes and reboot into safe mode with F8. Once windows loads, install MalwareBytes scanner and do a fullscan. You will need to reboot to kill all viruses, and probably have to run the full scan a second time in normal boot mode (not safe mode).

Then install a serious anti-virus anti-malware scanner, nothing from McAfee or Norton Symantec will be good enough, they are just shills for suckers. thank you internet."

"This error with lsass.exe file seems to be temporary, because when Windows throws this error in the startup when you shut down the computer and after 4-5 minutes when you start it again, it fixes itself. :) So it is a time sensitive issue.

I do not know the reason why this error occurs, but it might be interference with some software or virus to throw this error."