Use strong passwords
Using a strong password is important to ensure that it is not vulnerable to brute force attacks which are essentially repeated attempts at guessing a username and password. Avoid dictionary words, names, or simple variations of these. If you don't know what constitutes a good password: see the acceptable password guidance
Use anti-virus software
Set it to update automatically and run routine scans. UCL staff and students can get anti-virus software for their home computers for free. Several products are available and can be downloaded from the following link: http://swdb.ucl.ac.uk/
Regularly update operating systems and software
Set Windows to automatically download and install updates, you can manually check for updates by going to the following link: http://windowsupdate.microsoft.com
It is also important to update software installed on your computer, particularly Java and Flash. Your web browser needs to be up-to-date as a lot of malware infects computers through browsing, the following link is useful for checking if your browser and plugins are up-to-date: https://browsercheck.qualys.com/
Use a firewall
Anti-virus software often comes with a built in firewall, F-Secure does and is available for staff and students to download from the following link: http://swdb.ucl.ac.uk/
Make regular backups
Modern hard disks are very reliable and seldom fail. But seldom isn't the same as never, and there is always the possibility of accidentally deleting the wrong file.
You need to determine what files you're going to back up - think which ones would cause you inconvenience or upset if they were to disappear - and how often. At work central systems are backed up daily. This is almost certainly overkill for the majority of home systems, but it's a decision you have to make for yourself. Are you willing to spend a few moments backing up your correspondence each day, or prepared to risk a couple of days' work and perhaps tackle the task once a week?
Make your backups onto a separate device, not just somewhere else on your hard disk. (If the disk does fail and it contains all your backups, you'll likely lose the lot.) Remember also to check that your backups have worked every now and then. Pick a random document and see if you can get it back.