Open up regedit, go to HKEY_CURRENT_USER, Software, Microsoft, Windows, Shell. There will be a DWORD registry key named "BagMRU Size." This is the limit of the number of thumbnails to store for folder views. (It's set to something like 5000 as of SP2 in XP.) Open it up and change it to 100000 or something. Do the same for the key in HKEY_CURRENT_USer, Software, Microsoft, Windows, ShellNoRoam, which will be right underneath. Restart the comp and you're migrating thumbnails should go away.