See this article by Martin Walker from Sound On Sound, especially the last two paragraphs.
XP Tweaks For Music
Interesting. I also spotted this bit which was news to me:
Change Processor Scheduling to 'Background Services'
Navigate to the Advanced page of the System applet. Click on the Performance Settings button, select its Advanced tab and click on 'Background Services' for Processor Scheduling (see top pair of screens).
This is one of the few essential Windows XP tweaks, since it benefits the performance of ASIO drivers, which run as background tasks.
For anyone using ASIO drivers (and nowadays that includes just about every PC musician), this is the most essential tweak of all, because ASIO drivers run as background services in Windows. Music software and hardware developers Steinberg rely on this setting to ensure low latency without dropouts, and you may be able to run your audio interface at a significantly lower latency after this tweak.
So I did this. Result was ghastly on my system (my profile for details if you're interested) so I suppose the moral is to try out tweaks one by one and test their impact one at a time.
Thanks for the link, though.
edited for typo