Some people like to back up everything, even their desktop. It’s usually pretty easy to decide what you want to back up, but here are some guidelines. Back up content that is the most valuable, most irreplaceable or most troublesome.
Your most valuable content is likely your personal records and digital photographs. Your bank and financial information would likely be next. Then, think of content that would be a headache to install or upload again-like your music collection. What about content that you have paid for, like video, software packages, or more music. Then, consider content you’re working on. For some people, it is the newest most current content that is the painful to lose.
As for backing up your system files... we're techies here so we usually back up our system files. That said, most "normal" people probably wouldn't bother. The thing is, if your computer crashes and you need to re-install your system files, its a darn sight easier to do it from your backed up version than having to dig out a bunch of discs and do it the hard way. If you have lost the install discs, you may not remember which version you were running, making it hard to download from the manufacturer. So our two cents on system files is to back them up.