hi Leon,
shame it's still slow
. AVG is excellent for free, would be better than Microsoft's that you're using. Kaspersky is decent as well for $40 and I think that's with a 2 year license.
as was mentioned in the other thread, ctrl-alt-del and run task manager and look for any conspicuous programs running and or hogging cpu.
if there's any dodgy looking ones, Google the exact file etc and it should tell you if it's meant to be running or not.
also I thought Microsoft stopped updating xp after they realeased service pack 3, so I doubt you've had any updates for a few years.
matt