While Joel is, in general, correct, there are occasions when a problem leaves residues of the old installation, and these can on occasion make it impossible to install a new version properly. There is guidance in both the FAQ and the manual on completely removing Eraser (essentially, deleting the install folder, the Task List file, and the HKCU\Software\Eraser key in the Registry). Running a Registry cleaner after doing this may also help.
David