What should you backup is a personal question and I can’t answer it for you definitively, but you should backup any information that is important to you. Any data that you created, obtained, hard to find, contact lists, bookmarks, passwords, etc.
On a daily basis I backup the following:
- “My Documents” folder
- “My Images” folder
- “My Downloads” folder
- Internet Browser files such as a bookmarks, plugins, settings
- Email files such as files and settings
I also have a system setup for monthly and yearly backups which involve a different set of lists. You want to create a system to reduce the risks you identify (you can see a sample list of scenarios in my Why Backup Your Data post).