To me judging from what you've said (especially as you've said your PC is very old) it sounds like your PC can't manage some of the modern technologies that newer websites use (not 100% of your tech knowledge but in a nutshell as technology moves on the older stuff gets less compatible with the newer stuff - a lot of modern websites are now using html5 which can take its toll on older PC's both hardware/software side)
It could be anything from graphics card, browser, Operating System, Network adapter....
But have no fear, it could be fixed; if they did work in the first place it could be the case of a software update required on your PC or failing that the websites have been upgraded making them more hard work for your pc to deal with! Have you tried using a different web browser?
That can be the annoying thing with computers, it could be one of hundreds of possibilities haha
It must be said I'm no expert, but based on my knowledge from studying computing that is the conclusion I can make based on what you have said, but if I see the error message I can most likely turn my assumption into a conclusion