XP Logon changes to profile

I have 3 logon profiles with different thumbnail pictures for each but recently when I boot the machine all 3 thumbnails have been changed to 1 common picture. When I logon many of my desktop icons are missing and some programs are no longer associated with the profile and try to reinstall themselves. I have full system backups which cure the problem but only for 2 or 3 logons when the problem re-appears. I have virus protection and scans show no problems. Any ideas please.