Reinstalling Windows 98 SE

I have an elderly neighbor who refuses to give up on their Pentium 366mhz
computer. The original O/S was Win 95, which was upgraded to Win 98 and then
Win 98 SE. Recently they've switched from 56k dial-up to cable broadband,
the problem is that they do have Outlook Express installed on their PC.
I've tried searching Microsoft's Downloads for IE6 but I haven't been able to
locate it.

I do have a retail copy of Win 98 SE that I no longer use. Can I use my
copy of Win 98 SE and reinstall it over their multiple upgraded Operating
System to restore IE, Outlook Express, etc without overlaying or losing any
of their existing applications and data?? And would this solve their
problem??