Make sure your computer runs anti-virus software. If not, buy and install it immediately.
Even if you have this software, it must be updated regularly, as new viruses appear daily. Configure the programs to automatically download updates, making it easy to stay protected. Otherwise, periodically download the updates manually.
DO NOT OPEN an e-mail attachment unless you know who sent it. Even then, it's not totally safe, as a sneaky virus that has infected a friend's computer can access the e-mail address book, send a message to everyone, and attach itself. To be completely safe, scan the attachment with your anti-virus software BEFORE you open it.
If you receive a suspicious message, delete it immediately from your Inbox. When you delete a message, however, it's still on your system. Go into the Deleted Mail folder and delete the message again to permanently remove it from your computer.
Regularly back up your files. Should your system become infected, you won't lose valuable data.