Periodically the system file checker pops up a message to insert the Windows CD. If I do this, the disk is read briefly, but the message comes up again later. A look at the system logs reveals errors of this type (summarised here): -- event id=64004 The protected system file c:\windows\apppatch\aclayers.dll could not be restored to its original, valid version. The file version of the bad file is 5.1.2600.1106 The specific error code is 0x800b0100 [No signature was present in the subject. ]. -- Furthermore, the application log is full of entries occuring around twice a second, of source "ESENT", as follows: -- (event id=454) Catalog Database (700) Database recovery/restore failed with unexpected error -510. -- I've fiddled with the "esentutl" command, but I don't really know what I'm doing with it. Oh, another thing that might be related: I can't get any Windows updates (either from Windows Update or downloaded manually) to install; they always fail with messages about not being certified for Windows XP, etc. (that you get when installing some drivers). I'm guessing that somehow the verification system is broken or something? Anyway, I would appreciate any help on this issue; although there don't appear to be any viruses on the system, it's worrying to think that something strange may nonetheless be afoot... Any thoughts? (I've checked Microsoft's support site and found nothing particularly useful) Thank you thank you for any help :)))

I would reinstall "Service Pack 1".

You can download SP1a from this site: http://www.microsoft.com/windowsxp/downloads/updates/sp1/default.mspx

I downloaded SP1a, but when I tried to install it, an error message came up saying that the integrity of update.inf couldn't be verified, and to make sure that cryptographic servies were running. I restarted the Cryptographic Services service, and it still gave the same message. I'm guessing something to do with that service is the cause of the problem; can't verify files to restore, can't verify updates, fixes, etc., can't verify update.inf. Looks to me like its all part of the same problem, but unfortunately I don't know what the problem is :( Any further suggestions?