From M$:
Optimizing Startup Times by Using Defragmentation Tools
Windows XP Professional monitors the files that are used
when the computer starts and when you start applications.
By monitoring these files, Windows XP Professional 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 Professional
and start applications.
Prefetching is further improved if the files are located
next to each other on the outer edge of the disk. Windows
XP Professional optimizes the location of boot files and applications when the computer is idle. The optimization
occurs in the background and lasts only a minute or two;
you might hear the hard disk being accessed when
optimization occurs. After the initial optimization takes
place, subsequent optimization occurs, at most, every three days.
When you run the Disk Defragmenter tools that are included
with Windows XP Professional, they can perform any
optimization updates that are scheduled to take place
during the next idle period. The Disk Defragmenter tools
do not disturb the existing layout of optimized boot files and applications.
Note
* Computers running Windows XP Home Edition also prefetch
and optimize boot files and applications.
I have deleted all files there, even
Layout.ini, at times without problems.
It will rebuild itself over time.