Backup your stuff. No excuses. There are a lot ways to backup your stuff — CDs, DVDs, and USB "thumb drives" are cheap and easy. For large amounts of data invest in an external hard drive — often available for less than $100. They are easy to use, and most come with simple software for managing your backup files.
Keep your files organized. Don't put your your data files (documents, email, etc.) in the same place as the application programs (and vice versa). Windows users normally put their files in the "My Documents" folder. That makes it much easier to backup your important data.
Don't worry too much about backuping your entire hard drive. That's a lot of time and work, and unless you know what you're doing, you may not be able to use the backup as you wished. Most times it's easier to reinstall the system and your application programs, and restore just your data.
Keep in a safe place all of the CDs, DVDs, serial numbers and product codes that came with your computer.
In the end, there are only two kinds of computer users:
Those who have lost huge amounts of time, data and money; and