Having the OS prefetch data is great but if your system is a bit old it can actually slow down the system! I know on my dad's old P3 disabling the feature boosts WindowsXP performance nicely. To disable prefetching first load up regedit (Start -> Run then type regedit and press the ok button) and follow this path HKEY_LOCAL_MACHINE -> SYSTEM -> CurrentControlSet -> Control -> Session Manager -> Memory management -> PrefetchParameters and find the EnablePrefetcher binary value. >From there change its value to 0 then save and reboot.

Once that's done Windows will no longer prefetch data before you use it, it should make everyday computing faster but applications might take a bit longer to load. If you find that performance is worse than before simply change the EnablePrefetcher binary value back to 3.