An update on my progress-

I'm becoming more convinced that it's a hardware issue but I'm still
having problems working out exactly what the trouble is.
I ran a hardware diagnosic utiltity (Sis monitor or something). It
gave me warning's about the power supply voltage being low. '12V too
low' and '5V too low'. These values were reading lower than the
minimum operating range (according to this utility). The 5V range was
only a tiny bit lower than normal, the 12V was a bit more out.
The thing I don't understand is that my machine was working fine a
couple of months ago. Is it possible for a power supply to have a
partial failure like this? Or is it something on my motherboard? If
not, how could it have been working before? I haven't changed the
hardware configuration of my PC at all. This is the first time I've
run this hardware monitor so I don't know if the volatge was out all
along.

Could low voltage affect my PC in the way I've described in my
original post, i.e. choppy graphics update and more disk access than
seems normal? Another symptom is when I watch a video in media player
on full screen mode, I get the choppy refresh (though its fine if I
just make the window fill the screen by drag resizing).

This is really frustrating, does anyone have any ideas?

Thanks, Alex.