Years ago, only certain types of files (generally executable files like program files) could be carriers of a virus, however with the invention of macro languages in programs like Microsoft Word or Excel, and the use of Javascript in web page building, almost any type of data could theoretically carry a virus. The most common way a computer is infected with a virus today is through the use of a floppy disk containing data from another computer, or through the use of the Internet with email, IRC, or other online communication.
Because viruses are becoming more clever, no one should be using a computer that does not have anti-virus software running on it. The more you use a computer, the more likely it is you will encounter a virus.
Some common symptoms that could indicate your system's been infected are:
* Computer slows down without reason
* Unusual messages or displays on your monitor
* Unusual sounds or music played at random times
* Your system has less available memory than it should
* A disk or volume name has been changed
* Programs or files are suddenly missing
* Unknown programs or files have been created
* Some of your files become corrupted or suddenly don't work properly
