AVG Antivirus - a free anti-virus tool from the Germans. Be careful, they will try every respectable trick to try to get you to buy a paid versioin of their software but if you stick to your guns, you can use the free version and it is great.
Truecrypt - a free file encryption software program available for Mac, Windows and Unix.
Fileshredder - a free tool to securely delete your files so that they cannot be recovered. Any other way of deleting your files, including emptying your Recycling Bin, or even re-formatting your hard-drive can leave your personal data vulnerable.