There are some exceptions to this too such as AVG Virus Scanner
. However most free or cheap closed source applications want something in return for that low price. Perhaps they give you spyware, or they will start harassing you to buy a license. Even if this is not the case the coder can unintentionally create a vulnerability or fail to patch a vulnerability. Adobe Acrobat Reader is a free application supported by a major software company and even that has introduced vulnerabilities. Open source applications are exceptions because they can be maintained and critiqued by others. With all software though you have to justify the risk with the rewards. These applications also have to be updated, windows will not do it for you. Ubuntu will keep all your applications up to date which is a huge plus for linux security.6. Don't install extra applications.
When installing any application, use advanced install and un-check any offer to install additional applications. Even applications from Adobe and Apple ask to install something else, always say NO. If you are installing iTunes, don't accept Safari too. Don't install any software if you aren't going to use it.
7. Turn off nearly every program that is running near the clock.
Programs running in the background are a huge risk because they can be listening for or communicating with the internet without you knowing. Each one also takes a little speed away from the computer. While turning all all unneeded background tasks can require a degree in computers, most of us can get the tasks near the clock to stop running. Of course there are windows provided icons and your virus scanner that must remain. But you don't need that office toolbar, Adobe updater, etc.
. If you can not afford it, switch to 
$45
Reader/Writer $45
or 
if you don't know what truly makes it different in a practical manner. Your pictures will NOT look better and your wallet will be much lighter. The other rule is don't get sucked into megapixels. 3 Megapixels can make a decent 8x10 and after 5 megapixels it will be hard to tell the difference between that and an 8x10 from a similar quality 22 megapixel camera. The rebel XSi is an outstanding camera. It's pictures will look just as good as the pictures from a 50D or a 5DII which is why the others are a waste.
or 
. For photographers, all the lenses above are considered inexpensive. Some readers may notice a slight contradiction between my lens recommendations and body recommendations. Yes many people get outstanding pictures from the pure consumer lenses like 
or 
, etc. However if you really want to do some of the more creative work with photography you need to avoid buying your lenses at big box stores.