The links that are shown here are ones that I've found useful from time to time. Please let me know of any other links you think ought to be included.
For convenience, I've split what started as a long list into two parts - this one for OS X users and this one for PC users. Remember, whether you're a PC user or a Mac user, please do keep up-to-date with security fixes and other updates. If you're using OS X, you just need to click the Software Update item in the Apple menu. PC users need to choose Windows Update or Microsoft Update from the Programs list. Oh, and if you've converted your Mac system into a hybrid system by using Apple's Boot Camp or the Parallels software, you'll need to get all the PC security software to keep your system safe.