That older PC still has a lot of life in it. If you want to make if more zippy, in order of priority:

• Buy a decent graphics card.
  I had never realized the effect of a good graphics card on perceived performance. Those screens can pop right up (at least relative to what they were doing before).
• Memory, memory, memory
  its cheap and it makes a BIG difference. I recommend 64MB as a minimum for Win9x and 128M as a minimum for NT / Windows 2000. For NT/W2K performance I like to run 256M.
• Don't consume more than 60% of you available HD space. Get a defragger and run it periodically. As it get more fragmented, your PC will get slower and slower.
• Make sure your pagefile is large enough. I recommend 2x RAM.
• Keep fonts to a minimum. I have installed apps that load a sh*tl*ad of fonts and actually SLOWED my PC to nothing.