Hm.. I've never used Intuos5 before but it sounds like something could have corrupted sai to say the least. I wouldn't say a faulty disk would cause just one program to crash or mess up unless your computer is messing up entirely. Is your computer, not just your tablet or sai messing up? If things seem to be messing up in other places besides the art issues then I would do a system restore back to before you installed Intuos5, see if it really is Intuos5 or not.
well unfortunately with my mac computer, it wouldn't let me system restore at all. That's why I said I'll need to reinstall windows or take it to a shop. There was one other program that stopped working properly, VLC player, after I installed the tablet.
And I can now confirm that after doing a system restore on my laptop, SAI works fine again. So it really must be something about this tablet that has corrupted some of my files :/