A possible reason, just like in what I believe my case is with fan noise, is that there is always something running in the background services. When I initialy recieved my laptop if you left it alone a few seconds or you were only typing out for example a document the fan noise did not occur. Not due to many things being installed a lot of items will load in startup and stay there. If you go to Start -> Run and type in "msconfig" (without quotes) then startup you can count how many services have descided to startup on load.
There are also services in windows XP. Start -> Settings -> Control Panel -> Administrative tools -> Services. Dont remove any Microsoft ones you are unsure of. However some other serives may be added for example Norton can take up some system resources.
