Many paid cloud backup services are readily available (Carbonite, CrashPlan, etc.), but good free options are also available, including Google Drive (currently 15 GB for Drive, Gmail, and photos) and Microsoft SkyDrive (currently 7 GB, integrated as of Windows 8).
The immediate obstacle to using Google Drive or SkyDrive for backup is that both services sync only a single folder to the cloud, and it would normally be a hassle to keep all important stuff in the single sync folder, but fortunately there's an easier way to set up backup syncing in Windows: NTSF Junctions.
An NTSF Junction is a symbolic link to a folder, which has several advantages over a Windows Shell Shortcut (.lnk), including content sync by Windows SkyDrive software. (Shell Shortcuts will be synced, but not the content they point to.) So if, for example, you create a Junction to My Documents in your SkyDrive folder, then Windows SkyDrive will sync My Documents content to the SkyDrive cloud. If you keep your important items in My Documents, this will give you excellent online protection. You can add other folders to sync the same way with their own NTSF Junctions, even Desktop.
The primary limitation of this approach is that Windows SkyDrive does not monitor NTSF Junctions for changes the way it does the regular contents of the SkyDrive folder, so sync of NTSF Junction content must be manually initiated either by restarting Windows SkyDrive or by modifying something in the SkyDrive folder. (Regular sync could also be initiated with Windows Task Scheduler.)
So what about Google Drive? Unfortunately, as of this writing Drive doesn't support Junctions for content sync, so the only way to sync a folder like My Documents is to move it to the Google Drive folder. That's quite easy to do as described here and here, and works quite well in most cases.