First, check that the Esellerateengine.dll file that is in your C:\Windows folder is the last one, if unsure, you can download it here, and place there manually:
http://www.virtualisoftware.com/binaries/eSellerateEngine.dllNOTE that, the C:\Windows folder is the correct one ONLY under XP. Under Vista the correct folder should be:
C:\ProgramData\Esellerate
(C:\ProgramData is hidden by default, so you'll need to enable hidden files view in Explorer to see it). If the Esellerate folder doesn't exists, you can create it and put the dll there.
Also, check you don't have ANY other copy of the eSellerateEngine.dll in ANY other folder EXCEPT the correct one for your OS (which is C:\Windows under XP and C:\ProgramData\Esellerate). Look specifically into your FS9 root folder and your C:\Windows\System32 folder. NO eSellerateEngine.dll files should be present there.
You should have ONE and only ONE copy of this file (and this should be obviously the current version posted above), in the correct location for your OS.
All our installers automatically put the correct file in the correct place for your OS, but there might be other products out there using Esellerate that installis either an older version or the correct version, but in the wrong folder, or a combination of the two...