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Email is how most viruses and trojans are transmitted. Always have your virus scanner up to date. A good option is to use a virus scanner that integrates itself into the mail program you are using - such AVG, which will plug into Outlook.

Gmail is excellent for automatic scanning and filtering of spam emails as well. Be extra cautious with emails from a sender you don't know. A safe bet is to never open or double click on an email attachment that is of the format *.exe or filename.exe. If you know who sent it to you and you are expecting it and know what it is, then fine but otherwise, just dont open that attachment or if it's obviously spam, delete that particular email