Daily data back-ups using a little tool called mirror created a batch file and I click on it a couple of times a day.
This does all my data files, like documents, spreadsheets, emails, email account details, registration keys, address book, favourites/bookmarks, downloaded files/programs, etc. the list goes on and on but if you don't want to lose it back it up. There are many back-up programs that can simplify this task and run it every day.
I do a weekly 'full' image of my partitions and save them to a 2nd hard disk, they can also be saved to off-line storage, DVD, USB external hard disk, etc. as part of my weekly system maintenance.