First, it's not possible to help you, if you don't say anything about your system config. Are you using XP or Vista ? Because, it's entirely different.
In Vista, the correct place for the esellerateengine.dll is C:\ProgramData\Esellerate (this is an hidden folder)
In XP, the correct place is C:\Windows
Our installer detects the OS version and puts the file in the correct place but, another program that use Esellerate might have put the file in the wrong place, for example C:\Windows under Vista, which is wrong. If the dll is older, this is even more of a problem, because the one on C:\Windows is preventing any update.
This can usually be solved by manually removing the copy under C:\Windows and let the installer run again but, you need to have full admin rights to manually remove something under C:\Windows, because there's an additional protection (other than UAC) in Vista to prevent manually tampering the C:\Windows folder.
You can try rebooting and boot into Safe Mode (press F8 at start), then removing the esellerateengine.dll from C:\Windows in safe mode.