We have plenty of great how-to articles explaining different backup strategies and backup plans; but if external drives and thoughts of drive images make your eyes glaze over, they won't help. Instead, think about what you have on your PC that you'd miss if you lost it.
Backing Up Photos. If you have irreplaceable photos on your PC that you need to back up, but you don't want to spend the time or money required to back them up to a DVD or external drive, consider storing them in a Flickr account, a Picasa Web album, or even a Facebook album.
All of those options are free (though some services will charge for storage or monthly upload bandwidth beyond a specified limit), and they all have auto-upload functions to keep your photo backups going. Picasa users can do this automatically with Picasa Web Albums, while Flickr and Facebook users should check out Foldr Monitr for Flickr and LiveUpload to Facebook.
Using Dropbox. When it comes to documents, the main items I want to back up are my work-related documents (old article drafts, mostly), so I don't need to buy terabytes of storage. In fact, I don't even need to buy a USB flash drive. I simply signed up for a Dropbox Basic account--which gives users 2GB of free online storage--and copied my whole Work Stuff folder over to it.
That doesn't always work, however, because I have a handful of high-res image files scattered among the Word docs, and they would eventually use up all of the space in my free Dropbox account. Rather that shell out $10 a month or so to increase my space allotment, I periodically do a quick search for every Word file on my hard drive (by searching for *.doc) and drag the files into a new folder on Dropbox.
If you have other folders on your hard drive that you want to sync with Dropbox without relocating it, simply grab Dropbox Folder Sync. Also, since Windows treats Dropbox as it would any other folder in its file system, you could create a batch file with some basic DOS commands to automate that search and copy process.