Thanks for all of your replies.

I originally had a GeForce 4 ti4400 in the system, but that eventually died
and I replaced it with the cheapest card I could find - an ATI Rage
128Ultra. That card worked fine. I made sure to remove the drivers for that
card before installing the new one as I was aware that it could otherwise
cause conflicts.

My power supply is an Enermax 350W, which I am sure is more than sufficient.

Although it could well be an overheating problem, it irritates me that if it
is caused by this, why does the card not come with sufficient cooling
hardware? Why should I have to change the fan just to get the card to work??
It should work straight out of the box!

In reply to mr_buggerlugs, my pc does seem generally less stable with the
card in - not only does it reboot itself, a few programs are much less
stable now. e.g. outlook express and internet explorer crash frequently
(giving me windows errors, asking me to report the error), and sometimes msn
messenger.

Sapphire tech support (the only tech support that has actually bothered to
reply to my emails) have advised me to replace the card if the new Catalyst
drivers due out on the 7th January do not help the situation.