Check the sites/pages you are linking as some of those websites come with ads that automatically try to install spy ware and other annoying bits of junk (like Gator) when you are downloading pages. These bits of spy ware sometimes change the settings on your computer and cause the computer to crash frequently.
Some spy ware remains hidden in your computer and tracks all messages and key strokes on the computer. Most of these bugs are now coming along with certain commercials.
Also, once you visit any website on the Internet or post on a message board, you can be tracked. Some people naively believe that the Internet offers them invisibility but that is not so.
There are some free tools that can help.
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