What to Back Up
Your hard drive may contain hundreds of thousands of files. Many of them should be backed up every day, others only occasionally, and still others--including temp files, the hibernation file (hiberfil.sys), and your browser cache--not at all. Let's look at the different kinds of files individually.
Your documents: You should back up your word processing files, spreadsheets, and similar documents every day. Any basic backup program can perform incremental backups, in which the program copies only the files that have changed since the most recent previous backup. (Good backup programs also perform versioning; that is, they keep several iterations of the same file on hand and enable you to choose which version to restore.)
Your recent documents: If your backup program can handle incremental backups, you don't have to worry about recent documents as separate entities. But if you often work on these files on other people's computers, you may want to carry a copy of them on a flash drive or store a copy of them online.
Application data: Apps create and maintain data files such as e-mail messages, browser favorites, calendar entries, and contacts that require daily backing up. Most programs store them in a hidden folder inside your user folder (in XP, C:\Documents and Settings\your name\Application Data; in Vista, C:\Users\your name\AppData). Also, in XP, Microsoft stores Outlook and Outlook Express data in C:\Documents and Settings\your name\Local Settings\Application Data). Fortunately, any well-designed backup program intended for everyday, nonexpert users (as opposed to IT departments) knows where to look for Outlook data.
Media: If your backup medium is sufficiently roomy and fast, you can back up your photo, music, and video files every day. But these large files may require a separate backup strategy.
Heirlooms: Files that you want to keep forever--family photos, the special anniversary card you made for your parents, and so on--need backing up and extra protection.
Your system: You can always reinstall Windows and your apps, if you have the original discs or can download the programs. But if Windows becomes unusable or your hard drive crashes, switching to a system backup (also called a disaster recovery backup) that you create a couple of times a year can get your machine up and running smoothly without much effort.