It might be worth trying a reinstall then. If you can work you way through installing the mask add on you'll find it much easier to work with.
And, of course, you are in the extremely fortunate position that you can buy proper photoshop at a reasonable price, and that has masks built in (as does Photoplus for even less cash - but it's not CS, sort of halfway between Elements and CS).
You could always save the video to your computer somewhere, that way, you'll have it for reference.
Thanks for all your help Howard. When I used the programme last night, it still worked properly, so maybe the problem has been cured.
I am a bit worried about installing anything new on my laptop, as I'm running out of hard drive space. (I have problems discarding photographs that I know I won't use, 'just in case' so, even with my two external hard drives, I have to be careful not to overload the machine. That said, I may save your video on my largest external drive, so it won't clog up the laptop.
Still thinking about Photoshop proper. It's a massive programme with too many possibilities I think, but yet ...
By the way, I don't know how things are in France, but I did a five week 'introduction to Photoshop' course in January, which cost me appr £80.00 and even with that, I could get a student discount on Photoshop. Almost worthwhile to enroll for that alone!