Okay bagginsbill, I've followed your instructions, but something new has cropped up.

I've installed only the 'current' P8 content, to the runtime in my documents folder. I don't need the old stuff, it's in the P7 runtime.

On opening P8, the library had now dissapeared, and was replaced with a tiny message where the library should be, that I'd seen before, which is far too small to read. So this time I got a magnifying glass and it said something about "Flash, click here" so I did, and it asked me to download a recent update to the flash player. Though strangely I had already updated Flash Player yesterday! Anyway, I updated the player again.

The tiny message then dissapeared leaving a completely black area. So I restarted P8 and there was the library again, as a docked pallete in the top right corner. The library is working okay, and instantly indexed.

I exit P8, and everything else on the computer is ok. At this point nothing including P8 is running slow.