I discovered, by way of googling, that the .MSP and .MSI files are Microsoft update files, and can be removed with (apparently) no ill effect.

To make sure, I moved all the .MSP/I files to my backup HDD - just in case - and the rebooted.

The %free space went from 15% up to 55% !!! The .MSP/I files went back to when I bought the computer, and are the automatic updates and so on.

The files were in a hidden folder and the only way to see them was by opening up the explorer from within SequoiaView - that showed that about half the HDD space was taken up by these files.

I thought something weird was going on of late, as each time I defregged, there was less space. I appears that every time I went online, and an update appeared - more HDD space disappeared. I'll keep an eye out now for these update files, and delete them after installation.

So, with your help and a little bit of googling, the comp is now back to normal - or as normal as a PC can be.