sp2 takes more mem then winXP with sp1, i have a P4 3.0 GHz 512/ram, and i can feel from time to time my comp geting slow, and when i trying to find the problem, then i see my computer using 300+ ram, that means that it using more then 50% ram and it makes the computer acting like its slow, and i can only guess what that problem would do to a slow computer, but my first guess would be that it makes it freeze up, so my suggestion would be to remove SP2, then just use sp1 instead, sp2 was a beta not long ago so i belive it will be a sp3 soon that will take care of buggs in sp2, and if you havent done that yet, then try to tweak your xp a bit, this is what i have done to one of my slower computers:
Open Control Panel, dubble klick on the system icon. klick the Advanced tab.
Press Settings in Performance
Open Visual Effects, check Adjust for best performance.
Klick Advanced, check Background services. Klick OK
can be worth a try anyway, hope this solve your problems.. and yes, it affect mirc cos my mirc running smother when i tweak with this on the slow computer.. hope it helps..