What O/S are you running? If its XP then you'll to add more RAM, that will improve the performance significantly. Your question is way to vague and incomplete.
What motherboard do you have? Generally if you have the Load Optimal Values option in the BIOS that gets the settings pretty good. Then make sure DMA settings are enabled for the hard drives, memory settings are set to Optimal or Aggressive or Performance. For the O/s disable any unneccessary startup options, remove any spyware viruses, make sure you have all the latest driver installed for the motherboard and cards, such as video and audio.