Windows Movie Maker should do it, came as part of service pack 2 upgrade for XP, and now can only be downloaded from microsoft with service pack 2 (huge). seems to be available here:
http://downloads.cnet.co.uk/0,39100210,39059185s,00.htm
But first check to see if its on your mums puter, and check to see what operating system she is running. I had to add it to XP pro on thisen.