A friend had me check out his ailing Windows XP Home notebook computer last night. Every time he booted the computer, it would display an endless stream of error messages about wuauclt.exe having caused an access violation. This message would reappear every 30 seconds or so.
Microsoft has a hotfix (see 'related links', below) that is supposed to address this problem, but I was unable to install the hotfix - the system would display an error message saying that it could not verify the system configuration, and therefore could not install the hotfix.
As it turns out, the computer in question had suffered a series of hard disk failures - the system event log showed a series of disk bad block errors dating back over a month. So, I expect that critical files on the hard disk have been corrupted - including, perhaps, the Extensible Storage Engine (ESE) database or related files used by esent.dll.
The excellent GRC SpinRite hard disk test and repair tool located the bad sectors (SpinRite was unable to recover all the data, but at least the sectors are no longer available for use by Windows)
But what to do about the recurring access violation messages after logging in? In this case, I had to stop (and disable) the "Automatic Updates" service in the service manager. Once I did this, we stopped seeing the recurring messages - but, the computer will no longer check for Microsoft updates automatically.
Ultimately, I think we will need to run the Windows XP Home repair facility using the XP Home boot disks, which should fix the corrupted files. Unfortunately, I was unable to complete this process because I didn't have the XP Home SP2 boot disks handy.