Are you running Explorer 7 as well? Same thing happened to me today and I have no idea why or how to find it now.
Recently, I tried to access disk defragmenter from all program/accessories/system tools and discovered that they are not there any more. I can still access the tools from HELP and SUPPORT, so I know that they are still on my harddrive, but I am curious to know if anyone has any idea on how to restore them to their proper place in accessories. I am running Windows XP professional, service pack 1. Thank you for any suggestions, Lino