Ok, I don't think it is a RAM problem. RAM has tested ok. It happens a lot when
mirroring www.roadkil.net - I'll have to start taking note of what sites it
happens with, as I'm wondering if there is a way for a server to force a
program to close (maybe some sites don't like you mirroring them and somehow
send a close message to the program).

There are two other possibilities I think. The first being that Windows itself
forces the close for some reason (I had wondered if it was because the limit
on TCP/IP connections was being reached, but there is nothing in Event Viewer
at the time it crashed, the previous TCP/IP warning was about 20 minutes
earlier). The other possibility is a faulty control in WinHTTrack - the
program still creates all those 1KB files in the root of the C: drive when
mirroring (all start with the letter s, but otherwise seem to be randomly
named). I remember a post from Xavier saying that he couldn't fix it or didn't
know how to reproduce it or similar, but I think he did say that it shouldn't
be happening. It happens with the great majority of mirrors on every PC I've
used WinHTTrack on, and with multiple different versions of WinHTTrack, so
something is definitely amiss there - whether it is related to this random
closing problem, I don't know. My post mirror cleanup steps though always
involve deleting all those 1KB files WinHTTrack creates though, which is
always a pain in the backside to do.