Why my PC has become so much slow?

Hi,
I am quite confused about what has happened with my computer. When I begin to restart or shutdown my PC, it takes about 2-3 minutes for “Saving your settings” to shutdown, and then another 15-20 seconds for windows.
This is not the only case; even my system takes about 2-3 minutes to boot and load. I have an HP Pavilion, Win SP2, Pentium 4, 2.6 GHz, 1024 Mb RAM and 80 GB Hard Drive. Help needed.
Thanks!
