You could try adding 2gb of RAM to it. If you installed Leopard recently and noticed it slow down, it's because Leopard requires more RAM@512mb or quite possibly was installed over your old OS and that is causing the issue. So 1gb is recommended, but otherwise 2gb would help that much more depending on what you are doing with it. If it's running slow because you have many applications open or something similarly intense then that would be an issue.
If it's just arbitrarily running slow regardless of what you are doing, then you MAY have system errors or otherwise a hard drive issue. That's not always the case but is a possibility.
You can boot up from your system disk and run Disk Utility to repair disk/permissions and also download a free app such as Onyx or Cocktail that run some maintanence cleaning scripts on your system.
Or you can forego any utilities and boot up from your system disk and perform an Archive and Install (recommended if you recently installed Leopard, and didn't perform this or a clean install) while preserving user data to completely reinstall the base system (as it doesn't remove the old system you would need enough space on your drive). If you have a complete backup of all your data you can also just chose to completely format and do a clean reinstall of the OS, ensuring that everything is pristine and new.
If the problem persists after a format, then you've most liekly got a hardware issue on your hands.
