Two things to think about:
Is the avi player on the PC significanlty older than the QT version you encoded with. Could be whatever she's running on Win98 doesn't understand a later version of the codec used to encode.?
Other thing. I had the EXACT SAME PROBLEM (made a movie in QT that played fine on my Mac but was just a blank screen when playing on windows ...actually, the screen assumed "transparency" and showed whatever was behind the QT window). I finally got it to work with simple trick -- Grab the movie window and resize it smaller on the windows machine. Every time I did this, the movie content would miraculously reappear in the player window and play normally. (note this was with QuickTime player for Windows .. don't know if it would work with a different player). Strange behavior to be sure ... but it seems to work.
good luck
[edit] oops, sorry .. I just re-read your post. Is your problem being able to see the movie file on the CD in the first place ?