The consensus experiance with this problem is that resetting the Eudora Settings works to solve this problem.
See: Something Seems to be Covering the Ad (Windows)
Document ID: 2458HQ
http://eudora.com/techsupport/kb/2458hq.html
Resetting the Eudora Settings DOES NOT remove or change any of your mail account information.
Start up Eudora. Hold down the keys CTRL and SHIFT, then go to Tools > Options. You will be asked if you want to reset your settings file - click Yes
If this does not solve your problem, you have a similar related problem:
Eudora Forums > --== WINDOWS EUDORA ==-- > Technical Issues (crashing, error messages, migrating, etc.)
FAQ: Sponsored mode unable to get ads?
http://eudorabb.qualcomm.com/showthread.php?t=4064