I'm sorry to tell you but the snapping to the top of a cell when you scroll is a design choice by Microsoft that you simply cannot get around while working within Excel. You can middle click your mouse to enable smooth preview zoom with the mouse, but once you click the program, it will snap back.
Instead of looking for a plug-in, you should look for a different program to run the .xls files to work on them. Perhaps even something as simple as opening the document in Google docs might allow you to scroll normally.
That being said, if you are having the problem of the cells being so big that when you scroll down it skips over some cells, assign either a command button or keyboard shortcut to this macro:
Sub DownOne()
ActiveWindow.SmallScroll Down:=1
End Sub
I have used this on several occasions due to Excel skipping cells because of their size. The reason is that the mouse "single scroll down" is by default set to Down:=3
You might be able to map this macro to a mouse wheel scroll event, but I believe you need to add a .dll to use that event (ref: http://support.microsoft.com/kb/837910)