It is always a good idea to have a virus checker on your machine.
Viruses can come from a number of different places.
They can be downloaded off the Internet when you download a program or execute an attachement to an email. They can be transferred to your computer through a floppy disk that has been in another machine, or they can be transferred through the network. A good, recent virus checker can detect and warn you of potential viruses. There are a few good anti-virus programs available for free from the Internet. Check the list of good freeware and shareware sites.