Its worth google searching for the file name - in this case I got a couple of leads - either you can download the dll file from somewhere like this,
DLL dowloads, don't know what version its for or which directory it goes in in Win 98.
Or there will be some QandA site like this
Ask Angela - she thinks you should reinstall windows. (Its the last question on the page)
I would have thought you could reinstall OE from the windows setup tab (go start, settings, control panel, add/remove programs, windows setup). You might have to unistall it first and then reinstall? Do backup/copy your mail, account settings and address book first though.