Open the terminal ( Applications/Utilities/Terminal ) and type "top". That will show you what is hogging your processor time in order of the biggest to the smallest processor hogging program (the "%CPU" column shows how much of the processor's time is being taken by each program).
Are you sure that you have enough memory? If you start to run short, the effect on your computer's performance is staggering.
You should have at least 512 MB of memory. 1GB would be ideal.
Okay... 100 GB would be ideal... with a 1 TB hard drive. And a Porsche.
