Computer system is slowing down

I am confused with my computer, when starting and shutting it down it takes almost 5 minutes. For the windows, it takes 20 minutes to open. Even through booting the system, it takes 5 minutes to boot and load.
I am annoyed to wait every time I open my computer and the same time when closing it. I need help about this. I have HP pavilion, Win SP2, Pentium 4, 2.6 GHz,1024 Mb RAM and 80 Gb hard drive.
