You can do this by scheduling your backup job. Keep in mind that in order to set
this up you'll need a user account that has a non-blank password.

If you have Windows Backup still configured to start in the default Wizard mode,
click on Advanced Mode in the opening screen.
You should now see the Backup Utility. Click on the Jobs tab.
At the bottom of the calendar, hit the Add job button.
In the Backup Wizard, hit the Next button.
On the What to Back Up screen, select Back up selected files, drives, or network
data. Hit the Next button.
On the next screen, select the folders and files you want to back up.
On the next screen, select the location you want to store the backup file and
assign a name to your backup job. Keep in mind that you will not be able to
select your CD or DVD writer as a location to store the backup unless you have
packet writing software, such as Nero's InCD, Roxio's Direct CD, or Sonic DLA,
installed on your computer.
On the next three screens, select the appropriate options for your backup job.
On the When to backup screen, select Later, enter the job name, and hit the Set
Schedule button.
If you want to run this backup job manually, select Once. If you want the backup
job to run automatically, configure the schedule however you want.
Once you've set your schedule, hit the Next button.
Enter the password for your user account twice and hit the OK button.
Hit Finish.

To run this job manually, go to Control Panel -> Scheduled Tasks and right click
on the icon representing your backup job. Select Run from the menu.

If you configured this job to run on a set schedule, right click on the job and
select Properties from the menu. Click on the Settings tab. Put a check mark in
the box next to Wake computer to run this task. Keep in mind that your backup
job will not run if the computer is off.

For more info, click on Help in Windows Backup. Select Help Topics from the
menu. On the Index page, scroll down to Scheduling tasks. Click on backup jobs
and hit the Display button. You can also take a look at this article.

Windows XP Backup Made Easy
http://www.microsoft.com/windowsxp/using/setup/learnmore/bott_03july14.mspx

Good luck

Nepatsfan