Ok, so I was just about to post the question below, and then I realised the answer - offline sync files - When setting things up this morning I'd accidentally right-clicked and hit always available offline by accident. I thought I'd unset that, and maybe I had, maybe I hadn't, but windows had kept the offline copies. and of course windows keeps those on the system disc... (apparently very well hidden!)

So thought I may as well post just the original question in case someone ever searches for a similar problem.

original distraught plea for help follows:

I've just taken the plunge and bought an intel x-25v SSD. I got the old C Drive down to less than 20gb or so, and used the free trial of acronis true image to clone the image onto the SSD. Windows booted up first time, and I thought all hunky dory.

Even when I turned page file, hibernation and system restore all back on (I'd seen it recommended to turn them off before using acronis), the used space was still only 25gb or so.

But turned it on today and it's reporting only 300mb free.

The bizarre thing is though, I just can't see what is taking up the space - Treesize is an app I've always found helpful in the past for tracking down where the big space monsters are, but treesize says there's only 25gb being used. And if I select all in explorer and right click - properties, that reports only 25gb as well. (I have show hidden files and show protected system files on. I can see the pagefile.sys for example). But right click on the c drive and choose properties, and somehow 37gb is being used.

So I just can't work out what the hell is happening. Disc management reports only 250mb free now, 1% free space which tallies with what is reported as free space.