Thanks again for your replies!
My problem is that I’m not a Mac user, so I don’t know the basics.
She is using an iMac, looks pretty old… She has 1Gb of memory, Classic says there is not enough and it needs more than 128Mb, so there is obviously some thing screwed up.
Will this disc repair things that are wrong at the moment? Is there something similar to the System File Checker in Windows?