Most common answer is that the HDD needs defragmenting and possibly has disk errors. This can be made worse if you have a nearly full HDD. Check your task scheduler from the control panel administration link to see if you have things running during heavy activity, such as file indexing.
Sounds like a lousy os that doesnt optimize itself for hardware limitations.
Software can always abuse the hsrdware without hardly trying. The user should be the focus, not Stve Ballmers fantasies.