Ideally do this before you start using Project64, if you haven't, please read this page now anyway.

Make sure you have a clean and stable system, which means:

1. Have all the latest drivers intalled for all your hardware, especially video hardware.
2. Have as few applications running as possible - the emulator needs a lot of resources, the more you free the better it will perform.
3. Defragment your hard drive - this could speed ROM loading and improve swapfile performance.
4. Temporarily disable any virus checkers or scheduled tasks - if these start while you're playing performance could suffer badly

If you know your PC is not stable generally (maybe it locks up occasionally while playing PC games, or applications quit or crash for no apparent reason), stop right here and get it fixed! The emulator pushes system components hard and although it will not cause any damage, any existing stability problems that your PC may have are likely to show up when you start using the emulator