Whenever my Windows XP machine starts up, subfolders disappear from the first top-level folder, listed alphabetically (eg. from "C:\AA Backups"). The first time it happened I suspected user error (such as an unintentional delete or copy). But I then found it happens on every start-up, sometimes affecting huge numbers of files. Renaming the affected folder (eg to "ZZ Backups") just means that a different folder is affected the next time. Avast found no virus or malware that would seem to be responsible. The missing files are not visible to an undelete utility such as NTFSUndelete. Running "chkdsk/f" found no problems and did not fix the problem. File permissions also appear corrupted - a few files which should be accessible are missing "read" permission.

What's happened to this machine?? Any ideas or reports of similar experiences would be most welcome.