Thank you all for your advice. Even though it's too late at night to take it all in and prepare a decent reply to each of you, I can't trip off to bed without expressing my appreciation.
It will take me a while to take everything on board and decide what to try first, particularly since you seem to have the problem surrounded and propose attacking it from all sides! I notice that nobody has yet mentioned Lubuntu!
There don't appear to be any overheating issues anywhere, according to Speccy. Unlike in Scotland there are no fitted carpets in this house, only marnle and tiles with a few rugs, so there's much less fluff everywhere. I have tried Auslogics defrag. It was really fast on my own machine but more or less the same as Defraggler on the Presario and didn't recover the lost free space as I described in the original post - I have no understanding of what all that's about because I've only seen it on that machine with Vista.
I'm tickled by the account change trick which seems to fit in with popular wisdom on the strangeness of Vista. I didn't think that installing more RAM was a goer in the V6000 but will look again more closely. However, after my weekly efforts the computer runs much better and is acceptably fast but gets stuck in the mud again by the following Wednesday - all of which makes me think that, while there are many things which might usefully be improved, the big issue is the gradual loss of free space on the HDD (I don't understand that either). It may well be that the improvement after Defraggler (which was not found after using the Auslogics product) is perhaps not due to the actual defragmentation but rather to the recovery of free space. Tonight there is 36 GB worth of free space while at lunchtime there was only a tiny 22.5. During defragging today the free space worked its way down to 9.7 GB which was getting nearly too tight to guarantee successfully finishing the actual defrag process in less than a week but the recovery back up to 36 GB free was achieved at the end of the process as usual.
It's all mad and I'm glad that mine doesn't do that neither now nor before under XP.