Windows 7 - Boot drive missing ~20GB

I have noticed recently that around 20GB of space has just disappeared of my C drive, in the last week or so.
The drive is an 120GB OCZ Vertex SSD so that is a huge chunk.

How can I determine whats taking up this space? Disk cleanup was no help.
See below for the discrepancy.
Not sure where that 21gb for files comes from.

Thats about the size of a back up (or restore point) Use treesize professional (jam software) trial (free) to find out

I have restores disabled but found out the culprits, page file was enabled for 12gb and also hibernation was not off.
Once I've switched those 2 off all the space came back

You actually need both

Pagefile for if/when you computer crashes. It can only write the error if there is a pagefile = to the ram and on the same HD

Hibernation if you ever want to use hibernation or sleep it is needed.

Actually sleep works without hibernation on, just tried it there.
And the page file, what if I set for the hdd instead of the ssd?
Should be ok too right?

Nope. For BSOD's to be able to be written it has to be on the HD where the OS is.

You may be right abt hibernation but I remember some obscure tidbit from somewhere that is is nec.

If BSOD is the only reason then I don't care to be honest. Only an issue if its a repeating problem with BSOD.

Its not but you have made your mind up

Yeah will leave it off for now, if I get any unusual behaviour will allocate some.
With 12gb ram it shouldn't need to swap anything onto the hdd I think.