XP Pro running slow

Hi,

Does anyone know what causes the XP Pro to run slowly even after I tried to clear Cookies and Temporary Internet files or history?

The first time I install XP Pro, it was running fine and faster, but as I started installing and maybe keeping stuffs that I need in my hard drive, I started experiencing this problem. I have been installing trial programs and Uninstall them after a while. Do you think some of these trial programs can cause the system to run slow also?

My machine is Compaq Presario 6000.
Could anyone please give a suggestion on how to get rid of this problem
Your help is highly appreciated.

Thank you