In the control panel, you can select 'Administrative Tools > Services' and turn off some of those unnecessary services that run constantly. Just right click the ones that are not vital and select properties. Change the type to 'manual' from 'automatic' to prevent these services from starting up at boot time. Be careful not to turn off vital services like Windows and virus software components. Below is an old CNET article about turning off Windows XP services.
http://reviews.cnet.com/ipod/9602-12576_7-0.html?messageID=2505922
-SC

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