More RAM WON'T stop HDD activity
Turn off any background processes like Windows Update, Restore, etc.
The only downside is you have to remember to update your windows, antivirus, etc manually.
I also turned off "Optimize Harddrive when idle" with TweakUI and that helped a bit
Even with Swapfile off, I'm still seeing some intermittent HDD activity - I guess it's WinXP's nature