This one is every bit as obvious as the last one and has been drilled into us to the point that it should be impossible to forget. However, many times we are in a hurry, or our minds are on other things and we don't stop to consider the ramifications of running untrusted software. In my case, despite giving this same advice to countless friends, family members and colleagues, I was in a hurry and foolishly failed to heed my own advice, to rather embarrassing results.

Back up your system regularly.

Do yourself a favor. Go out and buy yourself an external hard drive from someone like Maxtor or Western Digital and dedicate it exclusively as a backup device. Then set up a backup application, such as Windows' Backup utility or the software included with these drives, and configure your system for regular backups.

For home computers, I would recommend a scheme such as a full backup once a week, with nightly differential backups in between. This will enable you to restore to the previous night's backup in a worst-case scenario, ensuring you never lose more than one day of work. And put a reminder on your to-do list to check your backup regularly to make sure that everything is in working order. After all, the worst time to find out your backup hasn't been working for the last month and a half is when you need to restore last night's data.

Make sure your antivirus and antispyware software are up to date and configured to scan files as they are accessed (active scanning).