The first thing you can do to any computer is increase the amount of RAM. The more RAM you have the faster Windows will run because it won't have to access the hard drive nearly as often to load parts of the OS or to swap running programs in and out of RAM to a swapfile on the hard drive.
You should also reduce the number of programs that are starting in the Startup folder and the system tray. The less programs that load on bootup, the faster the computer will run.
Finally, through Control Panel you can go into Advanced system settings, Performance, then Settings. Under the Visual Effects tab (with custom settings selected) you can un-check some of the effects. I suggest un-checking all of the effects that have the word "animate", "fade" and "slide", as these are visual effects that slow down the opening and closing of windows and such.