You probably have either an esellerateengine.dll duplicated file or, something else in your system blocked the installation of the dll.
First, check for duplicates here:
c:\windows\esellerateengine.dll
c:\ProgramData\Esellerate\esellerateengine.dll (this is only in Vista, and it's an hidden folder)
c:\YOURFS9Root\esellerateengine.dll
If you find any esellerateengine.dll file, manually remove it, and reinstall. Check your user account has admin permission.
If still doesn't work, try downloading the dll from here:
http://www.virtualisoftware.com/binaries/eSellerateEngine.dlland manually put it into C:\Windows, if you are on XP or c:\ProgramData\Esellerate\ if you are on Vista.