Backup My Documents – By default when you click on file > save as in an application it will default to saving the files in your my documents. So it is very important to backup my documents as this is where all of your personal data will sit.
Backup E-mail Files – Backing up your emails is very important, None of us would like to loose all of our e-mails. Outlook .pst files (The file where all your e-mails are kept) are known for becoming corrupt quite frequently, Also when they get to 2gig in size there is a high chance they will get corrupt.
Backup System Files – I dont think its very important to backup the system files, As long as you take an “Image” of your system I think this is enough. But it is always a good idea to backup everything if you can just to be on the safe side.
Backup Misc Data Files – You might have created a folder and keep files in there. If you have make sure you remember the location so we can back it up.
Backup System Registry – The windows xp system registry keeps all the information of your system, config / programs installed and so on. Sometimes you might go in to the registry and accidentally the wrong keys, or it is possible that the registry becomes corrupt.
Backup Desktop Icons – Do you save documents to your desktop?? If yes you will need to backup your desktop.
Backup Internet Favourites – Do you have many Internet favourites / bookmarks saved??
Backup Internet Cookies – You might want to backup these up as well, they store passwords for certain sites, It is possible you might delete the cookie then forget the password to your site.