How to authomatically back up files in my computer to my external hard disk?

Windows backup.

You need to install it from your XP home disk

If you use Windows XP Home Edition, you'll need to follow these steps to install the utility:

1.Insert your Windows XP CD into the drive and, if necessary, double-click the CD icon in My Computer.
 
2.On the Welcome to Microsoft Windows XP screen, click Perform Additional Tasks.
 
3.Click Browse this CD.
 
4.In Windows Explorer, double-click the ValueAdd folder, then Msft, and then Ntbackup.
 
5.Double-click Ntbackup.msi to install the Backup utility.

See:

http://www.microsoft.com/windowsxp/using/setup/learnmore/bott_03july14.mspx

http://support.microsoft.com/default.aspx?scid=kb;en-us;q302894


 

You also could run synctoy

http://www.microsoft.com/windowsxp/using/digitalphotography/prophoto/synctoy.mspx

"... if there is a setting or some script I could run so that when I save a file onto my laptop, it also then authomatically gets saved/updated to my external hard disk as well??? " ==> Not as you've described it (i.e. automatically backed up WHEN you save the file), but Windows Backup (noted above) or any of several other automatic backup utilities that can be run using the XP Scheduler will do the job quite nicely.

My personal favorite is SyncBack (http://www.2brightsparks.com/syncback/), which has a lot more options than NTBackup ... but ANY regularly run backup utility will give you the backups you need.   In addition to having SyncBack run automatically every night, I also have an icon on my desktop I can simply double-click to run the backup anytime I want (so if I've made a lot of changes I don't have to wait).

The key isn't which utility you choose to use (NTBackup, SyncBack, Karen's Replicator, and many others will do an excellent job) ==> but that you USE one :-)