Never boot from floppies that you have used in the labs. If there is a virus on them, you could infect the hard drive of another computer. The following are possible symptoms. Many can be explained by other causes besides viruses. If you notice several symptoms, however, you may have a virus.
Programs take longer than normal to load into the computer.
There is more disk drive activity. For example, disk drives run longer when doing a simple directory. Error messages occur regularly that previously did not exist. Less memory is available than usual. The executable size of programs has changed.