You can use Vista's Disk Cleanup utility to refresh thumbnails. Open the utility by entering "cleanmgr" in the Start Search box. When the Disk Cleanup Options window appears, click "My files only". Select the appropriate drive and click OK. Once the scan finishes, scroll down through the list and make sure the Thumbnails option is checked. Click OK to delete the currently cached thumbnails. They should now be recreated the next time you navigate to the folders.

I hope this helps.