Buy and use a reputable anti-virus package. The leading packages deal effectively with all but the most obscure viruses. But make sure the program is set up properly. Does it automatically scan floppy disks, CD-Roms and other removable drives? Does it monitor files over the Internet or local network? A virus package is secure only if it is kept current. Download updates from the Net, or subscribe to a disk-based update service.
Never copy commercial software from friends. This is illegal, and there is no guarantee that the files are clean.
Be circumspect about sources of shareware. To be safe, use a reputable shareware library or download it from the author's own site. Then check it with anti-virus software before using it.
Think carefully before accepting files from friends and colleagues. Does their level of anti-virus security match yours?
If you use the Internet, treat e-mails attaching unsolicited files with suspicion.
Buy virus software that checks all downloaded files, or don't download them at all, unless you know their origins.
Keep all disks for original software, and make a regular back-up of your filesn