You can use the built-in Back up utility in XP.
Go to Start>All Programs>Accessories>System tools>Back up
In the first window, choose "Advanced Mode". In the "Welcome" tab, choose
the
third option " Automated System Recovery Wizard " Click " next" ,
specify a folder for your back up data ,( I recommend to choose a
folder in a partition other than the place your XP is located ) .
Before clicking "Next" , You should put a Floppy disk into your computer.
This
disk will be used in an emergency case, so after back is finished, keep it
in a
safe place.
By clicking " Finish" the back up process starts. Later you can use this
file for undoing harmful changes to your computer. So keep in mind the place
of this file.