What is wrong with my macbook pro?

I am not really sure what has been going on with my macbook pro lately, but it is really starting to irate me. For starters, it lags big time even when my activity monitor says my cpu is running at or below 20%. My HD isn't full either- I have about 100g free. When I click on files/folders/options I have no response at times. For instance, I will right click on my trash folder and the menu box will be displayed "empty trash, open" which will stay frozen there regardless of where I click. Even if I wait 10 minutes the menu box will still be frozen there. I usually have to click about 50 times and press some keys THEN it decides to run every single thing i clicked/typed in sequence real fast. I have tried different mousers, and the same thing happens. Another problem is when I run my VLC player it opens up random black screens when movies are playing and expands them over the movie I am playing. I don't get it. Can anyone offer any good advice? I would appreciate it

 

I am running mac os x version 10.5.8 and here is all the other info
Model Name: MacBook Pro
Model Identifier: MacBookPro5,5
Processor Name: Intel Core 2 Duo
Processor Speed: 2.26 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 3 MB
Memory: 4 GB
Bus Speed: 1.07 GHz
Boot ROM Version: MBP55.00AC.B03