Some will format your hard drive, some will attach themselves to other programs making these programs unusable and some will attack the computer's memory. Most viruses are aquired from the Internet. You can get a virus by downloading files from your Unix account, the Internet, or from other people's computers. The best way to find a virus is to have a virus scanner on your computer and use it regularly. Command Anti-Virus is available at the UT Computer Store.
Most people scan for viruses weekly.
If you download a lot of files from the internet you might want to scan more often. First, don't panic. This means the virus scanner you have is working. Most virus scanners come with a way to disinfect your computer. After you have disinfected the computer, it is always good practice to reboot and scan it again.