I recently came accross a client who recieved this message when they tried to use a sharepoint (V3) list:
"Your client does not support opening this list with Windows Explorer. "
Obviously this isnt a very helpful message but after a short while of searching through events and web pages I found the following Microsoft Article which helped to resolve the issue. It is caused by a long URL or List name exceeding 100 characters.
Microsoft have noted the problem and now released a patch for download so you can clear this problem.
Support Link: http://support.microsoft.com/kb/923906