Interesting that you got a crash using 10.04 which you said worked
perfectly well before and it was 10.10 that that had the problems, or
was that a typo ??

Also interesting that changing the memory sticks gave *some* sort of
improvement but what does Ubuntu and WinXP say about the memory you
have installed now ?

BTW - did some more reading on the Dimension 2400 and whereas Dell say
that the slots can take up to 512Mb sticks, there are only 2 slots but
in the next paragraph they say maximum RAM is 2Gb, how that's supposed
to work is anybody's guess so there's a muck up somewhere. Also there
are comments about getting 1Gb sticks working or not so there seems to
be some sort of issue going above 512Mb per slot.

I'd be inclined to run with one memory stick at a time and see how it
goes regarding stability - you may find that some modules give lockups
and some don't. If so then memory timing differences may be throwing
Ubuntu into a spin whereas WinXP doesn't drive the memory as hard ?

To throw another item into the mix, the Dimension 2400 was made when
the infamous 'bad capacitor plague' was around so have you checked for
leaking electrolytic capacitors on the board ?