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I have used Windows XP Home Edition since 2004 and sometimes when the computer starts up and the Startup items have finished loading the hard drive begins to make a lot of noise and sometimes this may go on for 2 minutes. When I got my first computer I remember checking the CPU useage after reading reports of other people noticing high CPU activity levels but I did not find anything unusual.
If what you are experiencing is like what I describe and you do not notice any other problems I would think your computer is doing something it is designed to do. It may be doing a maintenance the operating system is designed to do.
Noticing you updated the Bios recently I would think the operating system is adjusting itself to the new Bios.
By Mark Russinovich http://technet.microsoft.com/en-us/sysinternals/bb842062.aspx
is free software applications that include an advanced Process Monitor which I have only opened to look at which may tell you which process is at work the next time you notice the computer activity.
The software is currently 27MB. I can only say I like the look of the System Internals Suite but it is free and works.