Computer Viruses by Symptom
Malware is often downloaded when you download a new program, software, or file on your computer. If you have downloaded malware on your machine, you may notice a slow down with your Internet speed or the speed of your applications. Your applications may also start crashing randomly, forcing you to reboot. Some malware will also force annoying and unexpected pop-ups.
A memory resident virus will install itself as part of the operating system when it is executed. Once it installs itself, it will overwrite functions, files, or disk sectors of the computer. These files can lay dormant until the computer owner activates them, but when they do become active, they can delete files, take up memory causing pop-up warnings that you are low on disk space, and corrupt files.
Browser hijacker viruses are often spread via free download and will hijack certain browser functions, usually in the form of redirecting the user to a particular site. One of the most common malware programs is called CoolWebSearch but plenty others are out there as well. The biggest sign that you have downloaded this type of virus will be a change to your default search page of the browser, home page, or an increased number of links to advertisement web pages.
If you think your computer is slower than normal or if you notice that files are missing, it would be a good idea to have your computer checked.