The main drive with the OS was getting quite full. At that, the page file allocation was on another drive already.
I added a new drive and moved a big directory onto it - leaving 45% free on the OS drive.
The OS drive was by then *very* fragmented.
So, I ran Defragger a couple of times. It shows a whole lot of randomly scattered gold-colored blocks which mean "Page File". Seems very odd and seems to get in the way of a good defrag. Defrag reports 6% but the drive map shows not nearly enough contiguous "Empty" space with all those "Page File" blocks in there.
I ran chkdsk
I ran Windows defrag. Which doesn't show the Page File blocks at all.
I ran Defragger again. Still shows the gold-colored Page File blocks all over the place. And the defrag seems hampered by something - so I suspect those Page File blocks.
There's a small DOS partition that has none of this and defrags very nicely.