As with many things on the Internet, pop-ups can be a double-edged sword. Pop-ups can be harmless or  they can lead to trouble. Unsolicited pop-ups can contain inappropriate content. Or you can unwittingly download spyware from a pop-up. These programs can monitor or control your computer use, and subject your computer to a barrage of pop-up ads, hijacked browsers, keys that don’t work, sluggish performance and other operating problems. To lower your risks, the Federal Trade Commission says to make sure your operating system and Web browser software are updated with free “patches” to close holes in the system that could allow spyware to invade your computer. Be careful what you download, and don’t click on links within pop-up windows. If you think your computer may have malicious spyware on it, experts advise you to get an anti-spyware program from a vendor you know and trust. Use it at least once a week -- and every time you start your computer, if possible. The software will detect spyware and allow you to delete it.