80-gig OS disk C close to maxing out - strategies?

Okay, another question while I wait for the replacement drive.

Last night, to my positive surprise, C had more than 5 gigs free. Which seemed weird because I had not touched anything, certainly not deleted any files, in 48 hours.

But then this morning, I was down to 856 Mb free. That was slightly less than the "living dangerously" level I had three days ago when this problem began.

So I'm hugely concerned how I could LOSE more than 4 gigs of free file space literally overnight. I'm also concerned curious about how I could GAIN 2 gigs of free file space like I did in the previous couple days.

I used ccleaner to delete a couple older restore points, which increased my free space by about 40 Mb. I ran Disk Cleanup, but that didn't find much to delete, certainly nothing in the gigabyte range.

Windows Update did two small updates yesterday. But I can't think of anything else I id that would have substantively changed the free space on my computer.

Ideas?

Finally got a new 250 gig Seagate drive in, which solves the space problem.

It's not the SSD that I received considerable encouragement to purchase from others here, but I'm not going to run out of space on it any time soon (again, this is a programs/OS drive only; everything else is on mirrored data drives). And it took my "disk data transfer rate" on the Windows Performance Index from a 4.9 to a 5.9, so I can give myself a half a high five for doing a little better than merely getting a larger disk.

Thanks, as always, to the ample help and comments that come from people on these forums.