For most windows user, they used to clean up windows junks to speed up system running. For the Intel Mac users running both Mac OSX and Windows, they may no ideal how to clean up their Mac OSX. Here is one way may useful for you.
It is very easy, you just need this free software "Ice Clean". So go to download it. If you can't google it. Here is the website you can get it.
Install the software by fellow the introduction. Make sure you download the right version for your Mac OSX. The Tiger 10.4 and Leopard 10.5 are using different version.
Start the ICE clean, and run deep system clean.
This software has more function other than clean up system. You can discover it, but for clean up system, it take a little bit longer than windows. DON'T force quite ice clean when it is running system clean up. It may damage your system.