I ran across the Intel Application Accelerator recently. This is a free Windows app from Intel that optimizes performance for Intel motherboards, especially unblocking disk I/O.
I installed this on my old 1ghz Dell and saw a dramatic improvement. Some video encoding stuff I was doing dropped from an average of five hours to around 2.5 hours -- amazing.
It only supports certain chipsets, so you have to read a bit first to make sure it won't hose your machine. (I got my chipset info from the Dell support site; I'm not sure of a way to determine it from Windows.) I'd recommend setting a system restore point before installing it just to be sure. (You can find System Restore in Start>Programs>Accessories>System Tools.)