In Windows XP, this is located in System Properties (Right-click on My Computer, go to Properties, Advanced, Performance Settings, Advanced). This setting depends on your computer and what applications are running. Typically, you would want to optimize for Programs, but, considering the fact that some plugins, ASIO, WDM, and audio/midi drivers may be considered background services, it may be worth a shot trying Background Services and see if your performance increases or decreases. It is doubtful that with processor speeds in the gigahertz range that this will have a drastic effect, but it may give a little boost.