To use it, go to the Backup and Restore Center (Start -> All Porgrams -> Accessories -> System Tools -> Backup):

In it, you can configure settings (in "Change settings) to make XP do backups to your HDA daily, weekly or monthly.

All you have to specify is that you want to backup to the network, e.g. to

The first time it runs it will back up all your files. Subsequent backups are incremental: only what changed since last backup is saved.

It saves your files in the form of zip files, to they are easy to recover, even without using the XP "Restore Files" program.