"Windows XP monitors the files that are used when the computer starts and
when you start applications. By monitoring these files, Windows XP can
prefetch them. Prefetching data is the process whereby data that is expected
to be requested is read ahead into the cache. Prefetching boot files and
applications decreases the time needed to start Windows XP and start
applications." This information is logged and stored on your hard drive
taking up space and requiring a process to be kept running monitoring which
applications are being run. This has a performance impact on your PC.
Disabling the Prefetch function or at least only enabling it for the Boot
Files will allow you to free up some system resources and preserve some disk
space.